fix: disable pthreads on GTest project top-level when using MinGW

[andoks]

in commit a634042 "Implement threading
support for gtest on Windows", testing of internal threading in google
test does not work on MinGW. The commit itself contains a comment
indicating that pthreads is not to be used under MinGW due to

// On MinGW, we can have both GTEST_OS_WINDOWS and GTEST_HAS_PTHREAD
// defined, but we don't want to use MinGW's pthreads implementation which
// has conformance problems with some versions of the POSIX standard.

The first comment on #363 might indicate the reason for this breakage.

This commit assures that pthreads is disabled on a top-level by cmake

Fix #606

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[KindDragon]

What are the problems? Link maybe?

#606

[andoks]

I signed it!

[googlebot]

CLAs look good, thanks!

[andoks]

@BillyDonahue: is the explanation I gave earlier good enough for this PR to be merged, or do I need to do more changes?

It will fix #606 as the commit message states, while the bigger pr #721 will improve the windows implementation and fix the issue by avoiding pthreads on windows althogether as far as I can read.

[KindDragon]

PR #721 merged, so you can close your PR