Respect CC & CXX env variables to set compilers

[smtrfnv]

• If CC and CXX environment variables are set then use them as paths to compilers.
• fix minor warnings appeared when clang++-15 was used
• remove not used compiler option from the configuration script

[mattrm456]

I think we could revise this PR to allow the user to solely set CC or CXX to a path where we find a compiler whose basename we recognize and set the other language compiler accordingly, if unset. For example if CXX is "/usr/local/bin/xlC_r" and CC is unset then we treat CC as if its set to /usr/local/bin/xlc. I can't imagine a scenario where one would want to use g++ for compiling C++ and clang for compiling C, to be linked together. This would be a small convenience feature so that users don't have to set both environment variables. For explicitly set paths to compilers whose basenames we don't recognize we'd treat both environment variables as mandatory.

That being said, in my experience it is uncommon for open source projects I've built to support such a convenience, and for the goal of this PR this proposed implementation is satisfactory. I'm leaving this comment for posterity in case we revisit this area in the future. Leave this PR as-is for now.

OK to merge.