Fix various srcdir!=builddir issues

Fix various srcdir!=builddir issues

jhbuild will default to srcdir != builddir in the future for various
reasons.

The enum generation rules were just broken and both did
cd $(srcdir) and $(addprefix $(srcdir)).

Also, remove an unnecessary $(addprefix) from the GIR sources; thanks
to Dan Winship for pointing out that make will look in both srcdir and
builddir, so it's not necessary to add a prefix explicitly.  Doing so
breaks obviously when adding the sourcedir to a builddir file.

https://bugzilla.gnome.org/show_bug.cgi?id=653199