Fix saving CFLAGS in LIBVIRT_DRIVER_CHECK_LIBXL - LIBVIRT_CHECK_LIB will
override old_CFLAGS, so use a different name.
Signed-off-by: NMarek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: NJim Fehlig <jfehlig@suse.com>