Scons: check for timespec_get on windows as well as unices

Dylan Baker requested to merge dbaker/mesa:submit/check-timespec-get into master

windows has a timespec_get function, and if we don't check that then it will try to fall back to using clock_gettime, which windows doesn't have.

Fixes: 68a785e6 ("Fall back on clock_gettime when timespec_get() is unavailable")

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