Linux: allow compile flags to be passed to AC_CHECK_LINUX_BUILD

Linux: allow compile flags to be passed to AC_CHECK_LINUX_BUILD

Some linux autoconf tests require particular compile flags such as
-Werror.  Add a parameter to the AC_CHECK_LINUX_BUILD macro that
lets the caller pass in any needed special flags.

Reviewed-on: http://gerrit.openafs.org/3769
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit ca38c9546e323b59189670b80740510fb9ec5269)

Change-Id: I8798e103b2aff77b87684b7fce56c730f0a74b8b
[andersk@mit.edu: Remove modifications of existing tests]
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-on: http://gerrit.openafs.org/3996
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Russ Allbery <rra@stanford.edu>