CMake: use threads if allowed and found, not just if found.

If the user's cmakelists.txt first look for threads using
find_package(Threads), then set(gtest_disable_pthreads ON),
and then include googletest. GoogleTest will not look for
threads. But since they have already been found before in
user's cmakelists, it will use them regardless.

This helped me fix build issue in darktable-org/rawspeed
on windows/MSYS2, even though there are threads, and they
are usable, googletest build was failing with issues
about AutoHandle. I was first looking for threads, and only
then including googletest, so no matter the value of
gtest_disable_pthreads, it failed.

The other obvious solution is for user to first include
googletest, and only then look for threads by himself.