fstests: update autoconf rules to define _GNU_SOURCE where needed

fstests: update autoconf rules to define _GNU_SOURCE where needed

Starting from xfsprogs commit cb898f157f84 ("linux.h: Use off64_t
instead of loff_t") in v4.5-rc1, fstests failed to build because of
configure error:

    checking xfs/xfs.h usability... no
    checking xfs/xfs.h presence... yes
    configure: WARNING: xfs/xfs.h: present but cannot be compiled
    configure: WARNING: xfs/xfs.h:     check for missing prerequisite headers?
    configure: WARNING: xfs/xfs.h: see the Autoconf documentation
    configure: WARNING: xfs/xfs.h:     section "Present But Cannot Be Compiled"
    configure: WARNING: xfs/xfs.h: proceeding with the compiler's result
    checking for xfs/xfs.h... no

    FATAL ERROR: cannot find a valid <xfs/xfs.h> header file.
    Run "make install-qa" from the xfsprogs source.

and that's because off64_t is not defined without _GNU_SOURCE being set.

Fix it by updating the autoconf rules adding _GNU_SOURCE define for the
following checks:

xfs.h libxfs.h xfs_log_format.h xlog_assign_lsn xqm.h xfsctl

Suggested-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>