cpp revisited: look for a cpp binary in various locations for use at runtime

cpp revisited: look for a cpp binary in various locations for use at runtime

Certain programs like dtlogin and tt_type_comp need access to the
system's cpp command to process certain files at runtime.  tradcpp is
not installed on most systems, and is intended for use as a imake-cpp
replacement which is all we are using it for.  We still need an
honest cpp for the CDE components to use.

Now, we look in various places for a 'cpp' command and set CPP_COMMAND
to it's value.  This way tt_type_comp and other CDE programs can do
required processing at runtime.