meson: Fix building of qmlgl plugin on Android and macOS
Needs gnustl for C++ STL support, which is the GNU STL on Android API 19 and older, and is a wrapper for the llvm-libc++ STL on newer APIs.
QtGui C++ templates use GL functions, so GLESv2 is needed at link time.
Tested with Cerbero.
Edited by Nirbheek Chauhan