adg: remove G_GNUC_CONST from get_type()

The _get_type() functions used to be G_GNUC_CONST, an ancient convention
adopted by old GTK+ releases.

They have side effects though, that of registering the new type. This
makes them unsuitable for the G_GNUC_CONST optimization.
cpml: remove G_GNUC_CONST from get_type()

Same rational from the previous adg commit.
build: remove gettext baggage

Good part of the crap in the po directory can be autogenerated by
configure, so removing it from the repository.

autogen.sh has been modified to avoid the call to gettextize: it
requires user interaction (probably on purpose) and this breaks
automatic builds triggered by git push (i.e., travis.org).