Fixes ArcticaProject/nx-libs#296.

 Inspired by the following X.org commit. Other than X.org, we will continue
 support for building nx-libs against libXfont1 for a while.

 commit 05a793f5b3c40747d5a92a076def7f4fb673c7e7
 Author: Keith Packard <keithp@keithp.com>
 Date:   Tue Sep 1 18:50:55 2015 -0700

    dix: Switch to the libXfont2 API (v2)

    This new libXfont API eliminates exposing internal X server symbols to
    the font library, replacing those with a struct full of the entire API
    needed to use that library.

    v2: Use libXfont2 instead of libXfont_2
Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>