Log Message:
rasqual: Fix installation if gtk-doc package is already installed

By default rasqual is configured with `--disable-gtk-doc'. However,
despite that, it will checks several `gtkdoc-*' tools anyway, ending
in a 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.

Noticed by <martin>, thanks!