add opengl_cflags to QEMU_CFLAGS

... and drop OPENGL_CFLAGS from Makefiles.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1490079888-29029-1-git-send-email-kraxel@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>