Define _LIBCPP_HAS_THREAD_API_WIN32 while building libcxxabi

When building libcxx, this is also already enabled via the cmake
option -DLIBCXX_HAS_WIN32_THREAD_API=ON.

This fixes building libcxxabi if pthreads are available.

build-libcxx.sh

@@ -103,7 +103,7 @@ for arch in $ARCHS; do
         -DLIBCXXABI_LIBCXX_INCLUDES=../../libcxx/include \
         -DLLVM_NO_OLD_LIBSTDCXX=TRUE \
         -DCXX_SUPPORTS_CXX11=TRUE \
-        -DCMAKE_CXX_FLAGS="-D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS" \
+        -DCMAKE_CXX_FLAGS="-D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS -D_LIBCPP_HAS_THREAD_API_WIN32" \
         ..
     make -j$CORES
     cd ..