Makefiles: Don't complain if depend file to be included doesn't exist.

While bootstrapping the dependencies, make will see the "include depend"
directive before the depend file has been created. To avoid a spurious
warning in this case we use "-include" instead, (which differs precisely
in the fact that it will not emit a diagnostic if the named file does
not exist).

src/glut/glx/Makefile

@@ -137,4 +137,4 @@ depend: $(SOURCES)
 	@ $(MKDEP) $(MKDEP_OPTIONS) -I$(TOP)/include $(SOURCES) \
 		$(X11_INCLUDES) > /dev/null 
 
-include depend
+-include depend

src/glw/Makefile

@@ -71,4 +71,4 @@ depend: $(GLW_SOURCES)
 		$(X11_INCLUDES) > /dev/null
 
 
-include depend
+-include depend