block: add gluster ifdef guard checks for SEEK_DATA/SEEK_HOLE support

Add checks to see if the system compiling QEMU has support for
SEEK_HOLE/SEEK_DATA.  If the system does not, we will flag that seek
data is unsupported in gluster.

Note: this is not a check on whether the gluster server itself supports
SEEK_DATA (that is already done during runtime), but rather if the
compilation environment supports SEEK_DATA.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Message-id: 00370bce5c98140d6c56ad5145635ec6551265cc.1475876377.git.jcody@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
