devel/anjuta: Fix build of SVN plugin when /usr/bin/svn exists on the system

The SVN plugin is never built and packaging anjuta fails. It links
with libsvn_client-1.so but the dependency is not properly declared.

A check for just 'svn' is not enough since the base system does not
provide subversion libraries. Change it to always use the libraries
from devel/subversion.

PR: 223046
Submitted by: tobik
Reported by: ohartmann@walstatt.org
Approved by: gnome (maintainer timeout, 2 weeks)