FindThreads: Skip check for -pthread flag when targeting the MSVC ABI

Since commit 3257c34073 (FindThreads: avoid failing in AIX when using
-D_XOPEN_SOURCE=500, 2022-04-30, v3.24.0-rc1~197^2) we no longer check
for `pthreads.h` before checking for the `-pthread` flag.  Compilers
targeting the MSVC ABI do not have such a flag, so avoid performing the
check unnecessarily.

Fixes: #23829