build: Relax requirement of gpg-error-config.
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 <34feb69f206f6fec69fb4bd79c126fcbde2c8e6a@fsij.org>

gpg/npth,gpg/npth