When no kernel source has been pointed to, first attempt to use

When no kernel source has been pointed to, first attempt to use
/lib/modules/$(uname -r)/source.  This will likely fail when building
under a mock (http://fedoraproject.org/wiki/Projects/Mock) chroot
environment since `uname -r` will report the running kernel which
likely is not the kernel in your chroot.  To cleanly handle this
we fallback to using the first kernel in your chroot.

The kernel-devel package which contains all the kernel headers and
a few build products such as Module.symver{s} is all the is required.
Full source is not needed.