bug#42181: 28.0.50; jansson library not available in Emacs 28.0.50 snapshot 2020-06-26 64-bit Windows installer

[Richard Day]

1. Start emacs with 'emacs -Q'
2. M-x eval-_expression_ (json-serialize '()) RET
3. Switch to *Messages* buffer, note message 'jansson library not found'

The jansson library is required for native JSON support which is
required for lsp-mode.  Without the library, lsp-mode does not work
correctly, specifically with clangd for C++ development.

Copying libjansson-4.dll from another project (e.g. mingw64) into the
bin directory and restarting emacs restores lsp-mode functionality.

It would seem that libjansson*.dll is missing from the Windows installer.


In GNU Emacs 28.0.50 (build 1, x86_64-w64-mingw32)
 of 2020-06-26 built on CIRROCUMULUS
Repository revision: ffb89ed5f07491e33fc79d8b4be49d9deba2ad4a
Repository branch: master
Windowing system distributor 'Microsoft Corp.', version 10.0.17763
System Description: Microsoft Windows 10 Enterprise (v10.0.1809.17763.1217)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark set
jansson library not found

Configured using:
 'configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install -C 'CFLAGS=-O2 -g3''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY W32NOTIFY ACL GNUTLS LIBXML2
HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS MODULES THREADS JSON PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: ENG
  locale-coding-system: cp1252

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: (only . t)

Load-path shadows:
c:/Program Files/emacs/x86_64/share/emacs/28.0.50/lisp/mail/metamail hides c:/Program Files/emacs/x86_64/share/emacs/28.0.50/lisp/obsolete/metamail

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
rfc822 mml easymenu mml-sec password-cache epa derived epg epg-config
gnus-util rmail rmail-loaddefs text-property-search time-date subr-x seq
byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs cl-lib
sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel dos-w32 ls-lisp disp-table term/w32-win w32-win w32-vars
term/common-win tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote threads w32notify w32 lcms2 multi-tty make-network-process
emacs)

Memory information:
((conses 16 46798 12186)
 (symbols 48 6051 1)
 (strings 32 16837 1949)
 (string-bytes 1 524687)
 (vectors 16 9288)
 (vector-slots 8 124392 10322)
 (floats 8 25 232)
 (intervals 56 214 0)
 (buffers 992 10))