Add template reference processing.

Add template reference processing.

Template references are words with double brackets, and refer to the
same field in the target pointed at the the double bracketed word.

For example, if a target's configuration has the following entry:

    'cflags' => '-DFOO {{x86_debug}}'

... then {{x86_debug}} will be replaced with the 'cflags' value from
target 'x86_debug'.

Note: template references are resolved recursively, and circular
references are not allowed

Reviewed-by: Andy Polyakov <appro@openssl.org>