try to improve mozjpeg test
try to improve mozjpeg test

save LIBS around test, test for ABI not header

see https://github.com/jcupitt/libvips/issues/431

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