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>