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/fluid/rules.mak

@@ -43,5 +43,5 @@ FLUIDCONF := \
 
 .fluid: fluidsynth toolchain.cmake
 	cd $< && $(HOSTVARS) CFLAGS="$(CFLAGS) -DFLUIDSYNTH_NOT_A_DLL" $(CMAKE) $(FLUIDCONF)
-	cd $< && $(CMAKEBUILD) . --target install
+	+$(CMAKEBUILD) $< --target install
 	touch $@