build: Relax requirement of gpg-error-config.

* npth.m4: Don't require AM_PATH_GPG_ERROR.

New gpgrt-config is completely optional. Configuring with old
npth-config script is still supported (with no gpg-error).
With new npth.pc file, developer can use PKG_CHECK_MODULES
in pkg.m4 or npth.m4 with AM_PATH_GPG_ERROR in gpg-error.m4.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>