When building for iOS in Cerbero, as of MoltenVK SDK 1.3.283, we have
to statically link to libMoltenVK since it no longer ships a dylib.
This requires linking to libc++, so we find the dep with the objc++
compiler to ensure that meson uses the right linker.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7091>