Fedora: Handle new kernel variant paths

With Fedora 20, Fedora now separates the variant from the rest of the
kernel version with a plus (+) instead of a period (.) . This results
in directories called e.g. 3.12.5-302.fc20.i686+PAE, where right now
we look for 3.12.5-302.fc20.i686.PAE.

Use this new directory scheme for Fedora 20 builds, so we can build
against non-default kernel variants on Fedora 20 and beyond.

Reviewed-on: http://gerrit.openafs.org/10620
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
(cherry picked from commit 837ec9dd41c4b1e10ad9d32a52b0f34dd665026a)

Change-Id: I513ab231a9d7b61ec7790eb99a27da698a355f17
Reviewed-on: http://gerrit.openafs.org/10622
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>