Warn when doing an out-of-source build and finding in-source build artifacts

Warn when doing an out-of-source build and finding in-source build artifacts

The reason to warn is that configuration *may* pick up on
configuration header files that are in the source tree, that might be
for a wildly different configuration than what is expected in the
current out-of-source configuration.

Reviewed-by: Matt Caswell <matt@openssl.org>