build: add convenience target 'build-sources'

This target allows to trigger a build of $(BUILT_SOURCES) manually. This
is handy if you tend to use 'make systemd-foobar' to directly build a
single binary. Those do not pull in $(BUILT_SOURCES), unfortunately. See
automake docs for that.

Makefile.am

@@ -6139,6 +6139,9 @@ hwdb-update:
 		http://standards.ieee.org/develop/regauth/iab/iab.txt && \
 	./ids-update.pl )
 
+.PHONY: built-sources
+built-sources: $(BUILT_SOURCES)
+
 .PHONY: git-tag
 git-tag:
 	git tag -s "v$(VERSION)" -m "systemd $(VERSION)"