Configure: Improve incremental build time

When Makefile/opensslconf.h is unchanged, don't write it at all.

Currently every time Configure is executed, these files are overwritten.
Makefile leads to regeneration of buildinf.h, and opensslconf.h is itself
a central header.

As a result, Configure triggers full rebuild, even if nothing is changed.

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