Backport googletest upstream patch to fix GCC/Clang warning

[Echelon9]

google/googletest#633

Fixes the following warning:

In file included from ../../lib/test/gtest/src/gtest-all.cc:42:
../../lib/test/gtest/src/gtest.cc:370:12: warning: missing field 'owner_'
      initializer [-Wmissing-field-initializers]
GTEST_API_ GTEST_DEFINE_STATIC_MUTEX_(g_linked_ptr_mutex);
           ^
../../lib/test/gtest/include/gtest/internal/gtest-port.h:1390:79: note: 
      expanded from macro 'GTEST_DEFINE_STATIC_MUTEX_'
    ::testing::internal::MutexBase mutex = { PTHREAD_MUTEX_INITIALIZER, false }
                                                                              ^
1 warning generated.

[Echelon9]

Merged.