Allow clib callable build flags

At the moment, we are guessing whether we have the MSVC compiler, by
looking at what Python was originally compiled for.  That works only if
we are using the same compiler, but this is not the case when we compile
with e.g. mingw-w64 using Python.org Python.

Unfortunately, at the time we are specifying build flags, we don't know
what compiler we are using.

Allow build flags to clib to be callables that return lists of strings,
instead of strings, where the callables can do work like inspecting the
compiler, at build time.

Use this to check for MSVC at build time, when specifying the
`/GL-` flag.

See gh-9977 for a related discussion about these flags.

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