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/libebur128/rules.mak

@@ -21,5 +21,5 @@ libebur128: libebur128-$(LIBEBUR128_VERSION).tar.gz .sum-libebur128
 
 .libebur128: libebur128 toolchain.cmake
 	cd $< && $(HOSTVARS_PIC) $(CMAKE) -DENABLE_INTERNAL_QUEUE_H=TRUE -DBUILD_SHARED_LIBS=FALSE
-	cd $< && $(CMAKEBUILD) . --target install
+	+$(CMAKEBUILD) $< --target install
 	touch $@