Fix VIPS_LIBDIR being "NONE" without --prefix
Fix VIPS_LIBDIR being "NONE" without --prefix

If the --prefix switch is not given to configure, the prefix variable is
NONE, so we need to the the default value from ac_default_prefix first.

jcupitt/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,lovell/libvips