osm_gps_map_image_get_zorder: Change return type to "gint"

... instead of "const gint".

The returned value is a scalar, not a pointer, therefore it's copied.
The "const" qualifier is unneeded and even breaks the build with older
GCC such as 4.2.

This fix was reported and committed upstream:
https://github.com/nzjrs/osm-gps-map/commit/a12c703180e0ea6c80a4781a084be470d006b498
Once again, fix build issue with libintl, GCC and *BSD

I still don't understand what's going on here, but this workaround seems
enough for now.

In the end, here's what we do on *BSD systems:
    o  clang: don't link with libintl
    o  GCC 4.2: don't link with libintl
    o  GCC 4.3+: explicitely link with libintl

This was tested on FreeBSD 9 & 10, with clang 3.2 (on FreeBSD 10 only,
using the default compiler), GCC 4.2 (base compiler) and GCC 4.6 (from
the ports tree).

The workaround is used on other BSD too, but honestly, I haven't check
if it was needed or if it works.