cleanup: warn about untested compiler versions (#7684)
cleanup: warn about untested compiler versions (#7684)

Add warnings when the library is configured with a compiler we do not
test with, and fatal messages for versions where we know our
dependencies won't work.

I also made some cosmetic fixes to the top-level CMakeLists.txt file,
just to make it more readable for us.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp