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.
[originally from svn r9270]