configure.ac not properly compiling pthreads support into googletest

[ngie-eign]

As I described in https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236169, googletest 1.8.1, compiled from ports, is not properly enabling pthread support in libgtest, due to an interesting set of assumptions made by googletest/m4/acx_pthread.m4, in terms of how arguments are passed to the clang compiler/linker. This might be triggered by clang++ incompatibilities with g++.

This is forcing some end-users of googletest to have to link libpthread support in after this fact, which is superfluous and unnecessary. FreeBSD is one example of this.

Please see the FreeBSD bug for more details.

CC: @asomers

[ngie-eign]

This is no longer an issue since the build systems supported are only bazel and cmake.