Instead of testing for different versions of GTK in increasing order

Instead of testing for different versions of GTK in increasing order
of preference so that the later ones overwrite the configured stuff
from the older ones, test in decreasing order of preference and stop
as soon as one is successful. Fixes a problem in which
autoconfiguration on a system containing only GTK 1 would go wrong
because the _failed_ test for GTK 2 would overwrite some but not all
of the variables set by the successful test for v1.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9270 cda61777-01e9-0310-a592-d414129be87e