Add --dependency-file=foo.d option to generate a gcc -M -MF style
dependency file for other build tools. The current output of
--generate-dependencies is only useful for use directly in Makefile
rules, but can't be used in other build systems like that.

The generated dependency file looks like this:
$ glib-compile-resources --sourcedir= test.gresource.xml --dependency-file=-
test.gresource.xml: test1.txt test2.txt test2.txt

test1.txt:

test2.txt:

test2.txt:

Unlike --generate-dependencies, the --dependency-file option can be
used together with other --generate options to create dependencies
as side-effect of generating sources.

Based on a patch by Tim-Philipp Müller in
https://bugzilla.gnome.org/show_bug.cgi?id=745754

The changes in this patch, compared to his are to always return
the hash table with file information from parse_resource_file, so
we can use it for dependency output, regardless if generate_dependencies
was TRUE or not.