Don't put .gitignore files under version control: they're generated by
git.mk.

Update git.mk and hook it up in Makefile.am.

Update remaining Makefile.am to get to a clean git status.

Don't clean/ignore files in $(srcdir) -- this makes no sense because
only generated files should be ignored, and generated files should be
generated in $(builddir).

https://bugzilla.gnome.org/show_bug.cgi?id=749706