Makefiles: Use VPATH to simplify dependencies

Currently dependency rules are defined using srcdir like this:
...
autogenerated-%.txt: $(srcdir)%.txt.in
...
to allow both for srcdir /a/b/:
...
autogenerated-%.txt: /a/b/%.txt.in
...
and for no srcdir:
...
autogenerated-%.txt: %.txt.in
...

A less verbose way to achieve this is to use VPATH:
...
VPATH = $(srcdir)
...
which allows us to use:
...
autogenerated-%.txt: %.txt.in
...

Add srcdir to VPATH and clean up dependencies.

Author: vries

Makefile

@@ -15,6 +15,10 @@
 #   along with this program.  If not, see
 #   <http://www.gnu.org/licenses/>.
 
+ifneq ($(srcdir),)
+VPATH = $(srcdir)
+endif
+
 .PHONY: all clean debug dump_gimple plugin show-ssa tarball \
 	test-suite testcpychecker testcpybuilder testdejagnu \
 	man
@@ -155,7 +159,7 @@ $(PLUGIN_OBJECT_GENERATED_FILES): CPPFLAGS+= $(if $(srcdir),-I$(srcdir))
 
 # This is the standard .c->.o recipe, but it needs to be stated
 # explicitly to support the case that $(srcdir) is not blank.
-$(PLUGIN_OBJECT_FILES): %.o: $(srcdir)%.c autogenerated-config.h $(srcdir)gcc-python.h $(LIBGCC_C_API_SO) autogenerated-EXTRA_CFLAGS.txt
+$(PLUGIN_OBJECT_FILES): %.o: %.c autogenerated-config.h gcc-python.h $(LIBGCC_C_API_SO) autogenerated-EXTRA_CFLAGS.txt
 	$(COMPILE.c) $(shell cat autogenerated-EXTRA_CFLAGS.txt) $(OUTPUT_OPTION) $<
 
 print-gcc-version: print-gcc-version.c autogenerated-EXTRA_CFLAGS.txt
@@ -173,10 +177,10 @@ clean:
 	cd gcc-c-api && make clean
 	find tests -name "*.o" -delete
 
-autogenerated-config.h: $(addprefix $(srcdir),generate-config-h.py configbuilder.py)
+autogenerated-config.h: generate-config-h.py configbuilder.py
 	$(PYTHON) $< -o $@ --gcc="$(CC)" --plugindir="$(GCCPLUGINS_DIR)"
 
-autogenerated-%.txt: $(srcdir)%.txt.in
+autogenerated-%.txt: %.txt.in
 	$(CPP) $(CPPFLAGS) -x c-header $^ -o $@
 
 # autogenerated-EXTRA_CFLAGS.txt is a byproduct of making
@@ -186,16 +190,16 @@ autogenerated-EXTRA_CFLAGS.txt: autogenerated-config.h
 # autogenerated-casts.h is a byproduct of making autogenerated-casts.c
 autogenerated-casts.h: autogenerated-casts.c
 
-$(PLUGIN_GENERATED_SOURCE_FILES): autogenerated-%.c: $(addprefix $(srcdir),generate-%-c.py $(GENERATOR_DEPS))
+$(PLUGIN_GENERATED_SOURCE_FILES): autogenerated-%.c: generate-%-c.py $(GENERATOR_DEPS)
 	$(PYTHON) $< > $@
 
-autogenerated-casts.c:  autogenerated-gimple-types.txt autogenerated-tree-types.txt autogenerated-rtl-types.txt $(srcdir)generate-casts-c.py
+autogenerated-casts.c:  autogenerated-gimple-types.txt autogenerated-tree-types.txt autogenerated-rtl-types.txt generate-casts-c.py
 	$(PYTHON) $(srcdir)generate-casts-c.py autogenerated-casts.c autogenerated-casts.h
 
-autogenerated-gimple.c: autogenerated-gimple-types.txt autogenerated-tree-types.txt autogenerated-rtl-types.txt $(srcdir)maketreetypes.py
-autogenerated-tree.c: autogenerated-tree-types.txt $(srcdir)maketreetypes.py
-autogenerated-rtl.c: autogenerated-rtl-types.txt $(srcdir)maketreetypes.py
-autogenerated-variable.c: autogenerated-gimple-types.txt $(srcdir)maketreetypes.py
+autogenerated-gimple.c: autogenerated-gimple-types.txt autogenerated-tree-types.txt autogenerated-rtl-types.txt maketreetypes.py
+autogenerated-tree.c: autogenerated-tree-types.txt maketreetypes.py
+autogenerated-rtl.c: autogenerated-rtl-types.txt maketreetypes.py
+autogenerated-variable.c: autogenerated-gimple-types.txt maketreetypes.py
 
 bindir=/usr/bin
 mandir=/usr/share/man

gcc-c-api/Makefile

@@ -15,6 +15,10 @@
 #   along with this program.  If not, see
 #   <http://www.gnu.org/licenses/>.
 
+ifneq ($(srcdir),)
+VPATH = $(srcdir)
+endif
+
 .PHONY: all clean check-api
 
 HANDWRITTEN_C_FILES = \
@@ -89,7 +93,7 @@ $(LIBGCC_C_API_SO): $(OBJECT_FILES)
 
 # This is the standard .c->.o recipe, but it needs to be stated
 # explicitly to support the case that $(srcdir) is not blank.
-$(OBJECT_FILES): %.o: $(srcdir)%.c $(GENERATED_HEADERS) ../autogenerated-EXTRA_CFLAGS.txt
+$(OBJECT_FILES): %.o: %.c $(GENERATED_HEADERS) ../autogenerated-EXTRA_CFLAGS.txt
 	$(COMPILE.c) $(shell cat ../autogenerated-EXTRA_CFLAGS.txt) $(OUTPUT_OPTION) $<
 
 autogenerated-casts.c: $(SOURCE_XML) xmltypes.py generate-casts-c.py