Detect scan-build the same way when trying to launch it and when
generating the target.
The detection method is:
1. look within SCANBUILD env variable
2. shutil.which('scan-build')
3. *on non-linux platforms only*: go through all the possible
name candidates and test them individually.
The third step is added following this comment
https://github.com/mesonbuild/meson/pull/5857#issuecomment-528305788
However, going through a list of all the possible candidates is neither
easily maintainable nor performant, and is therefore skipped on
platforms that should not require such a step (currently, only Linux
platforms).
This is a follow-up to the issue raised by @lantw44 during PR:
https://github.com/mesonbuild/meson/pull/5857
e7197895b2