redland: Fix installation if gtk-doc package is already installed

By default redland is configured with `--disable-gtk-doc'. However,
despite that it will checks several `gtkdoc-*' tools anyway ending
in an failure during the installation phase because only a couple
of them are available.

Add a kludge via CONFIGURE_ENV to avoid picking up gtkdoc-* tools.

Pointed out by <oster>, thanks!

(leot)