Make build compatible with GNU make 3.81 (#1246)

Author: LourensVeen

src/amuse_gadget2/Makefile

@@ -24,9 +24,12 @@ build_%:
 build_%/makefile_options: makefile_options_% | build_%
 	cp makefile_options_$* $@
 
-build_%/$(CODELIB) build_%/allvars.o &: build_%/makefile_options FORCE
+build_%/$(CODELIB): build_%/makefile_options FORCE
 	$(MAKE) -C build_$* -f ../src/Makefile -j $(CPU_COUNT) all VPATH=../src
 
+build_%/allvars.o: build_%/$(CODELIB) FORCE
+	@true
+
 
 # Load code configuration
 OPT :=