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.

Change-Id: I309272f4f4f740058c48e92530e40a1c64d63668
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>