Force libcxx to use win32 threads even if pthread.h happens to be available

Dockerfile

@@ -217,6 +217,7 @@ RUN cd libcxx && \
             -DLIBCXX_INSTALL_HEADERS=ON \
             -DLIBCXX_ENABLE_EXCEPTIONS=ON \
             -DLIBCXX_ENABLE_THREADS=ON \
+            -DLIBCXX_HAS_WIN32_THREAD_API=ON \
             -DLIBCXX_ENABLE_MONOTONIC_CLOCK=ON \
             -DLIBCXX_ENABLE_SHARED=OFF \
             -DLIBCXX_SUPPORTS_STD_EQ_CXX11_FLAG=TRUE \