Scons: check for timespec_get on windows as well as unices

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.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Part-of: <!8592>
35 jobs for !8592 with submit/check-timespec-get in 10 minutes and 55 seconds (queued for 4 seconds)
merge request