build-libcxx: Stop setting one unnecessary visibility define

Since libcxx includes libcxxabi with
_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS defined, the libcxxabi
declarations contain neither dllimport nor dllexport.

Earlier, we didn't use to include the libcxxabi headers with
_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS defined, but defining
_LIBCXXABI_BUILDING_LIBRARY instead. This caused libcxxabi
declarations to be marked dllexport. If any such functions would
end up emitted as part of the libcxx object files, this would
have been necessary to get them properly exported.

In practice libcxxabi functions aren't generated while included
in libcxx (they aren't templates and/or header inline functions),
_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS gets defined by the cmake
files, so we don't need to set this at all any longer.

build-libcxx.sh

@@ -132,11 +132,6 @@ build_all() {
         [ -z "$CLEAN" ] || rm -rf build-$arch-$type
         mkdir -p build-$arch-$type
         cd build-$arch-$type
-        if [ "$type" = "shared" ]; then
-            LIBCXX_VISIBILITY_FLAGS="-D_LIBCXXABI_BUILDING_LIBRARY"
-        else
-            LIBCXX_VISIBILITY_FLAGS=""
-        fi
         cmake \
             ${CMAKE_GENERATOR+-G} "$CMAKE_GENERATOR" \
             -DCMAKE_BUILD_TYPE=Release \
@@ -166,7 +161,6 @@ build_all() {
             -DLIBCXX_CXX_ABI_LIBRARY_PATH=../../libcxxabi/build-$arch-$type/lib \
             -DLIBCXX_LIBDIR_SUFFIX="" \
             -DLIBCXX_INCLUDE_TESTS=FALSE \
-            -DCMAKE_CXX_FLAGS="$LIBCXX_VISIBILITY_FLAGS" \
             -DCMAKE_SHARED_LINKER_FLAGS="-lunwind" \
             -DLIBCXX_ENABLE_ABI_LINKER_SCRIPT=FALSE \
             ..