LINUX: Honor --enable-checking for libafs

LINUX: Honor --enable-checking for libafs

When we build the kernel module on LINUX, we don't pass in any of our
CFLAGS, since the Linux buildsystem itself figures out what flags are
needed. However, this means that we don't pass in -Werror when
--enable-checking is turned on, so warnings may not cause the build to
fail.

To fix this, create a new autoconf variable, called CFLAGS_WERROR,
that only contains -Werror if --enable-checking is turned on. We then
pass that into the Linux module buildsystem, so -Werror is given to
the compiler when building our module.

Change-Id: I0f1ec8b1a8096d10642c67b86314604c20ea2c60
Reviewed-on: https://gerrit.openafs.org/13682
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>