Always use kbuild for all Linux kernel configure probes

Always use kbuild for all Linux kernel configure probes

Some Linux kernel probes for the existence of header files were done
with file existence checks (test -f).  This breaks if the kernel build
system is stacking multiple directories of headers together with
compile-time -I include path options, as is the case for the current
Debian Linux header packages.  Instead, always use kbuild to check
whether a kernel header is available.

Similarly, use AC_TRY_KBUILD instead of AC_TRY_COMPILE when checking
for an SELinux kernel, since AC_TRY_COMPILE doesn't call into kbuild
and won't get the correct kernel header paths.

This is part of the fix for Debian Bug#521745 and has been included in
the Debian package since 1.4.10+dfsg1-1.

Reviewed-on: http://gerrit.openafs.org/5
Verified-by: Russ Allbery <rra@stanford.edu>
Verified-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>