Start documenting Configure internals

This is a living document, everyone is encouraged to add to it.
Implementation details as well as broader implementation philosophy
has a place here.

I'm starting with documentation of the how conditions in build.info
files are treated.

Reviewed-by: Rich Salz <rsalz@openssl.org>

diff --git a/Configurations/INTERNALS.Configure b/Configurations/INTERNALS.Configure
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+Configure Internals
+===================
+
+[ note: this file uses markdown for formatting ]
+
+Intro
+-----
+
+This is a collection of notes that are hopefully of interest to those
+who decide to dive into Configure and what it does.  This is a living
+document and anyone is encouraged to add to it and submit changes.
+There's no claim for this document to be complete at any time, but it
+will hopefully reach such a point in time.
+
+
+----------------------------------------------------------------------
+
+Parsing build.info files, processing conditions
+-----------------------------------------------
+
+Processing conditions in build.info files is done with the help of a
+condition stack that tell if a build.info should be processed or if it
+should just be skipped over.  The possible states of the stack top are
+expressed in the following comment from Configure:
+
+    # The top item of this stack has the following values
+    # -2 positive already run and we found ELSE (following ELSIF should fail)
+    # -1 positive already run (skip until ENDIF)
+    # 0 negatives so far (if we're at a condition, check it)
+    # 1 last was positive (don't skip lines until next ELSE, ELSIF or ENDIF)
+    # 2 positive ELSE (following ELSIF should fail)
+
+Ground rule is that non-condition lines are skipped over if the
+stack top is > 0.  Condition lines (IF, ELSIF, ELSE and ENDIF
+statements) need to be processed either way to keep track of the skip
+stack states, so they are a little more intricate.
+
+Instead of trying to describe in words, here are some example of what
+the skip stack should look like after each line is processed:
+
+Example 1:
+
+| IF[1]                     |  1        |                               |
+|   ... whatever ...        |           | this line is processed        |
+|   IF[1]                   |  1  1     |                               |
+|     ... whatever ...      |           | this line is processed        |
+|   ELSIF[1]                |  1 -1     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ELSE                    |  1 -2     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ENDIF                   |  1        |                               |
+|   ... whatever ...        |           | this line is processed        |
+| ELSIF[1]                  | -1        |                               |
+|   ... whatever ...        |           | this line is skipped over     |
+|   IF[1]                   | -1 -1     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ELSIF[1]                | -1 -1     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ELSE                    | -1 -2     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ENDIF                   | -1        |                               |
+|   ... whatever ...        |           | this line is skipped over     |
+| ENDIF                     |           |                               |
+
+Example 2:
+
+| IF[0]                     |  0        |                               |
+|   ... whatever ...        |           | this line is skipped over     |
+|   IF[1]                   |  0 -1     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ELSIF[1]                |  0 -1     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ELSE                    |  0 -2     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ENDIF                   |  0        |                               |
+|   ... whatever ...        |           | this line is skipped over     |
+| ELSIF[1]                  |  1        |                               |
+|   ... whatever ...        |           | this line is processed        |
+|   IF[1]                   |  1  1     |                               |
+|     ... whatever ...      |           | this line is processed        |
+|   ELSIF[1]                |  1 -1     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ELSE                    |  1 -2     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ENDIF                   |  1        |                               |
+|   ... whatever ...        |           | this line is processed        |
+| ENDIF                     |           |                               |
+
+Example 3:
+
+| IF[0]                     |  0        |                               |
+|   ... whatever ...        |           | this line is skipped over     |
+|   IF[0]                   |  0 -1     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ELSIF[1]                |  0 -1     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ELSE                    |  0 -2     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ENDIF                   |  0        |                               |
+|   ... whatever ...        |           | this line is skipped over     |
+| ELSIF[1]                  |  1        |                               |
+|   ... whatever ...        |           | this line is processed        |
+|   IF[0]                   |  1  0     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ELSIF[1]                |  1  1     |                               |
+|     ... whatever ...      |           | this line is processed        |
+|   ELSE                    |  1 -2     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ENDIF                   |  1        |                               |
+|   ... whatever ...        |           | this line is processed        |
+| ENDIF                     |           |                               |
+
+Example 4:
+
+| IF[0]                     |  0        |                               |
+|   ... whatever ...        |           | this line is skipped over     |
+|   IF[0]                   |  0 -1     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ELSIF[0]                |  0 -1     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ELSE                    |  0 -2     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ENDIF                   |  0        |                               |
+|   ... whatever ...        |           | this line is skipped over     |
+| ELSIF[1]                  |  1        |                               |
+|   ... whatever ...        |           | this line is processed        |
+|   IF[0]                   |  1  0     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ELSIF[0]                |  1  0     |                               |
+|     ... whatever ...      |           | this line is skipped over     |
+|   ELSE                    |  1  2     |                               |
+|     ... whatever ...      |           | this line is processed        |
+|   ENDIF                   |  1        |                               |
+|   ... whatever ...        |           | this line is processed        |
+| ENDIF                     |           |                               |
+