Treat C++ flags more like C flags, and only if C++ compiler specified

C++ flags got the same config target value as C flags, but then
nothing else happened while C flags get all kinds of stuff added to
them (especially when --strict-warnings is used).

Now, C++ flags get the exact same treatment as C flags.  However, this
only happens when a C++ compiler is specified, to avoid confusing
messages about added C++ flags.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5181)