[python] correctly handle full path to compiler in CC and CXX env vars

[StrikerRUS]

For instance, CC can be /usr/bin/gcc. Refer to #2339.

@jameslamb Seems that the regex from #2339 for R-package will produce false positive result for something like C:/Users/gccharlee/pgcc-8 (any intermediate folder name which starts with gcc).

[jameslamb]

Ah! Forgot a $. Will make a PR momentarily, thanks for catching it.

[jameslamb]

Looks good to me! This does mean R and Python will have slightly different behavior (since you are using startswith() but I think it covers most cases and is definitely an improvement over the previous state.

[StrikerRUS]

@jameslamb Thanks for your review!

This does mean R and Python will have slightly different behavior (since you are using startswith() but I think it covers most cases and is definitely an improvement over the previous state.

Yeah, I decided to leave startswith because of its' better readability and simplicity in comparison with regex. And the most important - it doesn't require additional imports.

BTW, it's not critical since we run this compiler check only for macOS, but the regex from #2339 doesn't work for Windows (e.g. gcc.exe).