Import jansson-2.4 as wip/jansson.

Jansson is a C library for encoding, decoding and manipulating JSON
data. It features:

* Simple and intuitive API and data model
* Comprehensive documentation
* No dependencies on other libraries
* Full Unicode support (UTF-8)
* Extensive test suite

TODO:
Fails a self-test because the library exports more symbols than it should.
It seems "libtool ... -export-symbols-regex '^json_' ..." doesn't work,
at least not on NetBSD-6.99.20/amd64.
+ jansson.
+LICENSE=gnu-lgpl-v2.1