configure.in: Backport change to add support for WX_CONFIG as a preferred new replacement for WXCONFIG. Add wx2.8 and remove wx2.5 (development version) from the list of wx versions to check.
git-svn-id: file:///home/survex-svn/survex/branches/1.0@3441 4b37db11-9a0c-4f06-9ece-9ab7cdaee568