Fix building svn trunk on Mac OS X with the gnome keyring lib installed.

Don't check for gnome keyring and kwallet on Mac OS X. While these libraries are
available on the platform - you get libgnome-keyring when installing Gimp via
MacPorts - the auth providers' functions and symbols are currently not declared
on Mac OS X and Windows.

* configure.ac
* build/ac-macros/kwallet.m4: Don't check for kwallet and gnome keyring on
      darwin platforms.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1541698 13f79535-47bb-0310-9956-ffa450edef68