domd: Preserve Makefile time when it is unchanged

also on systems with makedepend that does not report its version, or that
its version does not contain "gcc" or "clang".

Some versions of makedepends just overwrite Makefile. Preserve the
timestamp of the previous Makefile, and copy it back if it is unchanged.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1370)