Add GTEST_HAS_PTHREAD to GTest::* target interfaces

[gitplcc]

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Add -DGTEST_HAS_PTHREAD=0/1 compiler flag to GTest::* CMake targets as pkg-config do.

Using CMake find_package my code compiled & linked smoothly but SIGSEGV when executed. Setting macro in CMAKE_CXX_FLAGS fixed it. I noted pkg-config gtest --cflags includes this macro.

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