Build: prevent deletion of autogenerated-*.c

Per comment from David Malcolm, deleting autogenerated-*.c files is
undesirable because it makes debugging more difficult.  Switch to a
static pattern rule to discourage Make from deleting these files.

Author: Kevin Pyle

Makefile

@@ -134,7 +134,7 @@ autogenerated-config.h: $(addprefix $(srcdir),generate-config-h.py configbuilder
 autogenerated-%.txt: $(srcdir)%.txt.in
 	$(CPP) $(CPPFLAGS) -x c-header $^ -o $@
 
-autogenerated-%.c: $(addprefix $(srcdir),generate-%-c.py $(GENERATOR_DEPS))
+$(PLUGIN_GENERATED_SOURCE_FILES): autogenerated-%.c: $(addprefix $(srcdir),generate-%-c.py $(GENERATOR_DEPS))
 	$(PYTHON) $< > $@
 
 autogenerated-gimple.c: autogenerated-gimple-types.txt autogenerated-tree-types.txt autogenerated-rtl-types.txt $(srcdir)maketreetypes.py