gtest: Fix compilation on MinGW with pthread /googletest#621

[stoffu]

Compile errors on Windows were reported by @iDunk5400 in #3245 (comment), but somehow the problem was never addressed since then.

In file included from C:/msys64/home/monero/tests/gtest/include/gtest/internal/gtest-internal.h:40:0,
                 from C:/msys64/home/monero/tests/gtest/include/gtest/gtest.h:58,
                 from C:/msys64/home/monero/tests/gtest/src/gtest-all.cc:39:
C:/msys64/home/monero/tests/gtest/include/gtest/internal/gtest-port.h:1782:3: error: 'AutoHandle' does not name a type; did you mean 'mutable'?
   AutoHandle thread_;
   ^~~~~~~~~~
   mutable
In file included from C:/msys64/home/monero/tests/gtest/src/gtest-all.cc:43:0:
C:/msys64/home/monero/tests/gtest/src/gtest-death-test.cc:637:3: error: 'AutoHandle' does not name a type; did you mean 'LocalHandle'?
   AutoHandle write_handle_;
   ^~~~~~~~~~
   LocalHandle
C:/msys64/home/monero/tests/gtest/src/gtest-death-test.cc:639:3: error: 'AutoHandle' does not name a type; did you mean 'LocalHandle'?
   AutoHandle child_handle_;
   ^~~~~~~~~~
   LocalHandle
C:/msys64/home/monero/tests/gtest/src/gtest-death-test.cc:644:3: error: 'AutoHandle' does not name a type; did you mean 'LocalHandle'?
   AutoHandle event_handle_;
   ^~~~~~~~~~
   LocalHandle
C:/msys64/home/monero/tests/gtest/src/gtest-death-test.cc: In member function 'virtual int testing::internal::WindowsDeathTest::Wait()':
C:/msys64/home/monero/tests/gtest/src/gtest-death-test.cc:656:36: error: 'child_handle_' was not declared in this scope
   const HANDLE wait_handles[2] = { child_handle_.Get(), event_handle_.Get() };
                                    ^~~~~~~~~~~~~
C:/msys64/home/monero/tests/gtest/src/gtest-death-test.cc:656:36: note: suggested alternative: 'wait_handles'
   const HANDLE wait_handles[2] = { child_handle_.Get(), event_handle_.Get() };
                                    ^~~~~~~~~~~~~
                                    wait_handles

...

As a quick workaround, I've applied the un-merged upstream patch google/googletest#621 (deemed as duplicate of google/googletest#721). But it seems that the vendored gtest codebase is periodically updated by @fluffypony, so maybe this patch is not the right way to handle this issue.