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/spatialaudio/rules.mak

@@ -23,5 +23,5 @@ spatialaudio: spatialaudio-$(SPATIALAUDIO_VERSION).tar.bz2 .sum-spatialaudio
 .spatialaudio: spatialaudio toolchain.cmake
 	cd $< && rm -f CMakeCache.txt
 	cd $< && $(HOSTVARS) $(CMAKE) -DMYSOFA_ROOT_DIR=$(PREFIX) -DHAVE_MIT_HRTF=OFF
-	cd $< && $(CMAKEBUILD) . --target install
+	+$(CMAKEBUILD) $< --target install
 	touch $@