build-libcxx: Remove some hardcoded flags for libcxxabi

These are included properly from libcxx's __config (or __config_site)
now.

build-libcxx.sh

@@ -187,7 +187,7 @@ build_all() {
         if [ "$type" = "shared" ]; then
             LIBCXXABI_VISIBILITY_FLAGS="-D_LIBCPP_BUILDING_LIBRARY= -U_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS"
         else
-            LIBCXXABI_VISIBILITY_FLAGS="-D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS"
+            LIBCXXABI_VISIBILITY_FLAGS=""
         fi
         cmake \
             ${CMAKE_GENERATOR+-G} "$CMAKE_GENERATOR" \
@@ -212,7 +212,7 @@ build_all() {
             -DLIBCXXABI_LIBDIR_SUFFIX="" \
             -DLIBCXXABI_ENABLE_NEW_DELETE_DEFINITIONS=OFF \
             -DCXX_SUPPORTS_CXX_STD=TRUE \
-            -DCMAKE_CXX_FLAGS="$LIBCXXABI_VISIBILITY_FLAGS -D_LIBCPP_HAS_THREAD_API_WIN32" \
+            -DCMAKE_CXX_FLAGS="$LIBCXXABI_VISIBILITY_FLAGS" \
             ..
         $BUILDCMD ${CORES+-j$CORES}
         cd ..