configure: Fix kernel devel check.

According to Michal Marek <mmarek@suse.cz> grepping the Makefile for
VERSION and PATCHLEVEL is not supported and these variables may be
removed int the future.

The correct way to get kernel version from the build directory is to run
'make -s kernelversion'.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>