Fix race condition on parallel builds

From sm64pc/sm64ex#512 , fixes issue https://github.com/Llennpie/Saturn/issues/21

Author: Llennpie

Makefile

@@ -249,7 +249,7 @@ endif
 endif
 
 # Make tools if out of date
-DUMMY != make -C tools >&2 || echo FAIL
+DUMMY != make -C tools -j1 >&2 || echo FAIL
 ifeq ($(DUMMY),FAIL)
   $(error Failed to build tools)
 endif