Log Message:
libidn2: Fix installation if gtk-doc package is already installed

By default libidn2 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.

Pointed out by <scole> via PR pkg/52667.