build: Provide npth.pc, generated by configure.
* configure.ac: Generate npth.pc. * Makefile.am (pkgconfigdir, pkgconfig_DATA): New. (EXTRA_DIST): Add npth.pc.in. * npth-config.in: Use @PACKAGE_VERSION@. * npth.pc.in: New.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>