Support dynamic loading of libjansson on MS-Windows

* src/json.c [WINDOWSNT]: Define fn_* function pointers to jansson
functions.
(json_delete) [WINDOWSNT]: A wrapper around fn_json_delete, needed
by json_decref.
(init_json_functions) [WINDOWSNT]: New function.
(Fjson_serialize, Fjson_insert, Fjson_parse_string)
(Fjson_parse_buffer) [WINDOWSNT]: Call init_json_functions if
needed, and record JSON in Vlibrary_cache.
* src/emacs.c (main): Don't call init_json on WINDOWSNT.
* src/w32fns.c (syms_of_w32fns): DEFSYM "json".

* lisp/term/w32-win.el (dynamic-library-alist): Add JSON DLL to
the list.

* configure.ac (HAVE_JSON): Empty JSON_LIBS for MinGW.

* nt/INSTALL.W64:
* nt/INSTALL: Add information about libjansson.

1 files changed, 7 insertions, 0 deletions

diff --git a/nt/INSTALL b/nt/INSTALL
index 30e14293f5c..361d607ff68 100644
--- a/nt/INSTALL
+++ b/nt/INSTALL
@@ -806,6 +806,13 @@ build will run on Windows 9X and newer systems).
   Prebuilt binaries of lcms2 DLL (for 32-bit builds of Emacs) are
   available from the ezwinports site and from the MSYS2 project.
 
+* Optional support for JSON
+
+  Emacs can provide built-in support for JSON parsing and
+  serialization using the libjansson library.  Prebuilt binaries of
+  the libjansson DLL (for 32-bit builds of Emacs) are available from
+  the ezwinports site and from the MSYS2 project.
+
 
 This file is part of GNU Emacs.