Build files: parametrize cpp

Instead of having the knowledge of the exact flags to run the C
preprocessor only and have it output on standard output in the deeper
recesses of the build file template, make it a config parameter, or
rely on build CPP in value ('$(CC) -E' on Unix).

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/5356)