libjq autoconf goo

Here it is, finally.

BTW, note that main.c uses jv_mem_alloc() and jv_mem_free(), which means that libjq needs to export those, but I don't think there's any reason that main.c should use jv_mem_alloc(), nor that libjq should export it (nor jv_mem_free()).

TODO:

 - Add autoconf goo to install headers...
 - ...which means making sure we end up with just one (or two) headers (<jq.h> and <jv.h>, or maybe <jq/jq.h> and <jq/jv.h>).