Add httpd server updates.

Add httpd server updates.

* httpd/server.cxx (server::access_handler): Save regexp matches.
  (server::queue_response): Add response headers.
* httpd/main.cxx: Add initial support for build info.
* httpd/Makefile.am: Link with the json-c library.
* httpd/server.h (struct request): Add 'matches' variable.
* configure.ac: Add the json-c library as a requirement of the httpd
  server.
* config.in: Regenerated.
* configure: Ditto.
* httpd/Makefile.in: Ditto.