Add JSON API for Asterisk.

This provides a JSON API by pulling in and wrapping the Jansson JSON
library[1]. The Asterisk API basically mirrors the Jansson
functionality, with a few minor tweaks.

 * Some names have been asteriskified to protect the innocent.
 * Jansson provides both reference-stealing and reference-borrowing
   versions of several API's. The Asterisk API is exclusively
   reference-stealing for operations that put elements into arrays and
   objects.
 * No support for doubles, since we usually don't need that.
 * Coming along for the ride is the ast_test_validate macro, which made
   the unit tests much easier to write.

 [1]: http://www.digip.org/jansson/

(issue ASTERISK-20887)
(closes issue ASTERISK-20888)
Review: https://reviewboard.asterisk.org/r/2264/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@378915 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configure.ac

@@ -406,6 +406,7 @@ AST_EXT_LIB_SETUP([INOTIFY], [inotify support], [inotify])
 AST_EXT_LIB_SETUP([IODBC], [iODBC], [iodbc])
 AST_EXT_LIB_SETUP([ISDNNET], [ISDN4Linux], [isdnnet])
 AST_EXT_LIB_SETUP([JACK], [Jack Audio Connection Kit], [jack])
+AST_EXT_LIB_SETUP([JANSSON], [Jansson JSON library], [jansson])
 AST_EXT_LIB_SETUP([KQUEUE], [kqueue support], [kqueue])
 AST_EXT_LIB_SETUP([LDAP], [OpenLDAP], [ldap])
 AST_LIBCURL_CHECK_CONFIG([], [7.10.1])
@@ -1846,6 +1847,8 @@ AST_EXT_LIB_CHECK([INOTIFY], [c], [inotify_init], [sys/inotify.h])
 
 AST_EXT_LIB_CHECK([JACK], [jack], [jack_activate], [jack/jack.h])
 
+AST_EXT_LIB_CHECK([JANSSON], [jansson], [json_dumps], [jansson.h])
+
 # BSD (and OS X) equivalent of inotify
 AST_EXT_LIB_CHECK([KQUEUE], [c], [kqueue], [sys/event.h])