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/gme/rules.mak

@@ -19,5 +19,5 @@ game-music-emu: game-music-emu-$(GME_VERSION).tar.bz2 .sum-gme
 
 .gme: game-music-emu toolchain.cmake
 	cd $< && $(HOSTVARS_PIC) $(CMAKE) .
-	cd $< && $(CMAKEBUILD) . --target install
+	+$(CMAKEBUILD) $< --target install
 	touch $@