ENH: Detect clang++ as a g++ alternative on Linux

[mark-thm]

Before

pytensor will only attempt to use g++ as a C++ compiler when running on Linux.

After

Improve pytensor by:

1. If the CXX environment variable is set, use the value to determine the C++ compiler
2. Discover g++ or clang++ on any platform

Context for the issue:

Today, pytensor will only discover g++ on Linux even though it can/will use clang++ on MacOS.

I'd be willing to propose a change here but wanted to gauge maintainer interest, first.