build-sys: remove unused USE_COMMON_DOC_BUILD

Remove from gnome-hello since eaa2917 and gnome-common since a912a74b.
build-sys: s/GNOME_ENABLE_DEBUG/AX_CHECK_ENABLE_DEBUG

As recommended by gnome-common macro documentation.
build-sys: update git.mk & to latest usage
build-sys: Use AM_GLIB_GNU_GETTEXT

It fixes intltool po/Makefile.in.in being overwritten by autoreconf autopoint.
build-sys: fix make distcheck

Update icon-theme.cache on uninstall, based on gtk-demo Makefile.am
build-sys: remove extra AC_SUBST

This is not needed