Improve depfile generated by glib-compile-resources

glib-compile-resources --dependency-file= currently generates a depfile with rules that look like this:

foo.xml: resource1 resource2

This means that if any of the files listed in the GResource manifest foo.xml change, rebuild foo.xml because foo.xml depends on those files. This is not useful because the XML manifest is not expected to be a generated dependency and even if it was, changes to the listed files would not imply any need to regenerate the manifest. What we really do need to regenerate is the C source file that is generated by glib-compile-resources after processing the XML manifest and all the resource files. That is, the rule should look like this:

foo.c: foo.xml resource1 resource2

as suggested by Hans Ulrich Niedermann in the issue report.

Fixes #2829 (closed)