environment: Fix INFOPATH for *BSD ports and pkgsrc

Ports and pkgsrc are widely used on *BSD systems to install third-party
software, but these systems patch all software to install Texinfo pages
in $prefix/info instead of $prefix/share/info. In order to set correct
INFOPATH on prefixes managed by ports or pkgsrc, we check whether
$prefix/info exists and use it if $prefix/share/info cannot be found.

https://bugzilla.gnome.org/show_bug.cgi?id=782598