Backport change from 1.2.0:
survex.spec.in: Update survex.com URLs to drop "www.". Update
wxGTK dependency to >= 2.6.0. Reenable "BuildRequires:" as
the issue with building the package on Debian is no longer
relevant. 2.6.0 may not actually be required for Survex 1.0.x, but
we've not tested with 2.4 in ages, and it's unlikely to still be in
use.