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.