with MSVC, _fix_compile_args(include_dirs=None)
appends to self.include_dirs  on each call.

Shouldn't be modified, but hard to tell if it is.

Allow lazy creation of the DSOs list, which
can avoid unnecessary (~slow) probing work
when running eg. './setup.py sdist'.

Helps with GHA, where conditional values are easier than
conditionally unset.

Maybe simplify/speed up PIP installs

More convenient to override the whole platform name,
including architecture.