Bug 630812 - Ignore gnome-doc-utils, .dirstamp and GSettings files in git.mk

Bug 630812 - Ignore gnome-doc-utils, .dirstamp and GSettings files in git.mk

Patches from Philip Withnall.

[PATCH 1/3] Ignore gnome-doc-utils' .mo files and generated headers

gnome-doc-utils can optionally generate a header (specified in DOC_H_FILE)
which is cleaned by gnome-doc-utils.make in mostlyclean. Additionally, it
generates MO files for the documentation translations which are also cleaned
by gnome-doc-utils.make in mostlyclean.

[PATCH 2/3] Ignore .dirstamp files

These are automatically removed on distclean by automake under some
circumstances.

[PATCH 3/3] Ignore files generated by gsettings.m4

gsettings.m4 generates .valid files and can optionally generate enum files,
which are all removed on mostlyclean by gsettings.m4.