Update configure help msg to match actual defaults

Update configure help msg to match actual defaults

The defaults for LINUX_KERNEL_PATH and LINUX_KERNEL_BUILD in
acinclude.m4 were changed in 2cfd611, 94ff565 and 3f9d982 without updating
the output of ./configure --help. Change the description of
linux-kernel-headers and linux-kernel-build to show the correct defaults.

Reviewed-on: http://gerrit.openafs.org/8759
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 986a8541518fb03a69cf5ece2055cb9cd50bda60)

Change-Id: I2fc463fc3b5793d6125f7367de22631c8e52da21
Reviewed-on: http://gerrit.openafs.org/8805
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>