Configure script output doesn't mention CXX

Bug report

Bug description:

I was having trouble getting test_cppext to pass on both my Mac and Ubuntu laptops. (Oddly enough, while running that test in isolation, it passed.) It succeeded on my Raspberry Pi. Trying to figure out what I'd done wrong, I noticed that I was setting CC=clang except on the Pi, but wasn't specifying a C++ compiler. I don't know what logic configure uses to set the C++ compiler if CC is set, but figured I could probably override it. ./configure --help doesn't mention anything C++-related, but CXX is referenced in configure.ac.

Thinking I'd hit on the solution by specifying CXX=clag++, I tried again. Alas, I still get the test_cppext failure and will continue to hunt for that problem. Nevertheless, it seems to me that ./configure --help should mention the CXX variable.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS, Linux

Linked PRs

• gh-157046

The issue points to configure.ac and ./configure --help; start by reading the CXX handling in configure.ac and checking the generated help output. Done means the help output documents CXX; PR gh-157046 is already linked, so check its status before starting.