npth.m4: Assume new gpgrt-config.
npth.m4: Assume new gpgrt-config.

* npth.m4: Don't supply CC but use --libdir.

Signed-off-by: NIIBE Yutaka <34feb69f206f6fec69fb4bd79c126fcbde2c8e6a@fsij.org>

gpg/npth,gpg/npth