build: Fix srcdir != builddir for gresources

There are three fixes here:

1) We need to specify --srcdir=$(srcdir) for the generate-dependencies
   invocation; otherwise it won't know where to find the files that
   the resource file references.
2) We need to prefix the resource file with $(srcdir) in the $(shell)
   call, because 'make' won't automatically search for it there.
3) We DON'T need to prefix the resource file with $(srcdir)
   in the build command when using $<, because 'make' will search
   VPATH for it and automatically prefix it.

Yes, this is all confusing.