Generate EXTRA_CFLAGS before recursing into gcc-c-api

Rules in gcc-c-api assume the available of
autogenerated-EXTRA_CFLAGS.txt, so ensure it is generated before
recursing into gcc-c-api, even in highly parallel workloads.

Author: Kevin Pyle

Makefile

@@ -150,9 +150,12 @@ $(PLUGIN_DSO): $(PLUGIN_OBJECT_FILES) $(LIBGCC_C_API_SO)
 	    $(LIBS) \
 	    -lgcc-c-api -Lgcc-c-api -Wl,-rpath=$(GCCPLUGINS_DIR)
 
-$(LIBGCC_C_API_SO):
+.PHONY: gcc-c-api/phony-stamp-gcc-api
+gcc-c-api/phony-stamp-gcc-api: autogenerated-EXTRA_CFLAGS.txt
 	$(MAKE) -C gcc-c-api libgcc-c-api.so CC="$(CC)" TARGET_GCC="$(TARGET_GCC)"
 
+$(LIBGCC_C_API_SO): gcc-c-api/phony-stamp-gcc-api
+
 $(PLUGIN_OBJECT_GENERATED_FILES): CPPFLAGS+= $(if $(srcdir),-I$(srcdir))
 
 # This is the standard .c->.o recipe, but it needs to be stated