Add runtime version checking functions

This patch adds two new exported functions:

* `jansson_version_str` - Returns a human-readable version number
* `jansson_version_cmp` - Returns an integer less than, equal to, or greater
  than zero if the runtime version of Jansson is found, respectively, to be
  less than, to match, or be greater than the provided major, minor, and micro.

doc/apiref.rst

@@ -58,6 +58,25 @@ the library:
       /* Code specific to version 1.3 and above */
       #endif
 
+Additionally, there are functions to determine the version of Jansson at
+runtime:
+
+.. function:: const char *jansson_version_str()
+
+   Return the version of the Jansson library, in the same format as
+   the ``JANSSON_VERSION`` preprocessor constant.
+
+   .. versionadded:: 2.13
+
+.. function:: int jansson_version_cmp(int major, int minor, int micro)
+
+   Returns an integer less than, equal to, or greater than zero if
+   the runtime version of Jansson is found, respectively, to be less
+   than, to match, or be greater than the provided *major*, *minor*, and
+   *micro*.
+
+   .. versionadded:: 2.13
+
 ``JANSSON_THREAD_SAFE_REFCOUNT``
   If this value is defined all read-only operations and reference counting in
   Jansson are thread safe.  This value is not defined for versions older than