loggen: fix build vs. newer xfsprogs headers

loggen: fix build vs. newer xfsprogs headers

v2: just include xfs/libxfs.h for the conftest prerequisites

commit cc085d770adb in xfsprogs removed some header files that are
included by loggen.c. Add an autoconf test that checks to see whether
xfs_log_format.h is present. If it is, include that instead of the
deprecated headers and add an alias for XFS_TRANS_MAGIC since that
value no longer exists.

With this patch, xfstests builds on both f20 and rawhide (f21) boxes.

Signed-off-by: Jeff Layton <jlayton@poochiereds.net>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>