Add a magic vpath to the unix makefile.

This satisfies the Makefile's dependency on itself when buiding
in a parallel directory, like I use for the mingw builds, without
having to 'touch Makefile' as a work-around.

unix/Makefile

@@ -207,6 +207,8 @@ clean:
 	-rmdir ${WORKDIR}
 
 # Make everything depend on changes in the Makefile
+# This vpath directive needs to be before any include statements
+vpath Makefile $(dir $(lastword $(MAKEFILE_LIST)))
 ${ALL_ASM} ${ALL_OBJS} ${ALL_DEPS} ${ALL_TARGETS} : Makefile
 
 # Specify which targets are phony for GNU make