Replaced elaborate install logic with just a bunch of symlinks.

Maybe I'm naive but even after thinking about it for quite
some time I can't see why it should hurt to just build the
cc65 binaries from the sources "in place" and have 'make
install' simply create some symlinks in usr/local/bin.

The new Makfile builds the binaries with builtin search paths
matching their build location. So the symlinks in usr/local/bin
allow to allow to use them out-of-the-box without additional
environment variables.

src/Makefile

@@ -28,7 +28,29 @@ mostlyclean:
 clean: mostlyclean
 	$(RM) -r ../bin
 
-.PHONY: all $(PROGS) mostlyclean clean
+install: all
+	$(foreach prog,$(PROGS),$(INSTALL_recipe))
+
+uninstall:
+	$(foreach prog,$(PROGS),$(UNINSTALL_recipe))
+
+.PHONY: all $(PROGS) mostlyclean clean install uninstall
+
+##########
+
+define INSTALL_recipe =
+
+ln -s $(abspath ../bin/$(prog)) /usr/local/bin/$(prog)
+
+endef
+
+##########
+
+define UNINSTALL_recipe =
+
+$(RM) /usr/local/bin/$(prog)
+
+endef
 
 ##########