m4: readline: Add gross kludge for include path

m4: readline: Add gross kludge for include path

Unfortunately the data reported by pkg-config is not completely
accurate, so until the issue has been fixed in readline we need
to work around it in libvirt.

The good news is that we only need the fix to land in FreeBSD
ports and macOS homebrew before we can drop the kludge, so
we're talking months rather than years.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>