xfsprogs: resolve Debian readline build issue

xfsprogs: resolve Debian readline build issue

Address the recently reported build issue with libreadline5/6, via
the gplv2 route.  Since this appears to be a relatively recent pkg,
I made its use conditional so the deb build continues to work for
everyone not running a bleeding edge distro.  Works For Me (tm).

This addresses Debian bug 553875: libreadline5-dev removal pending

Signed-off-by: Nathan Scott <nathans@debian.org>
Signed-off-by: Alex Elder <aelder@sgi.com>