contrib: Allow cmake make invocation to communicate with the jobserver

Since make can't ensure $(CMAKEBUILD) will invoke $(MAKE) itself, the sub-make process wasn't able to communicate with the jobserver, causing jobs to run with a single job instead of however much are available.

contrib/src/soxr/rules.mak

@@ -42,5 +42,5 @@ endif
 		-DWITH_OPENMP=OFF \
 		-DWITH_AVFFT=ON \
 		-Wno-dev
-	cd $< && $(CMAKEBUILD) . --target install
+	+$(CMAKEBUILD) $< --target install
 	touch $@