This patch fixes a bug and related test failure on platforms where 'char'
is unsigned.

Taken from upstream:
https://github.com/open-source-parsers/jsoncpp/commit/f11611c8785082ead760494cba06196f14a06dcb

diff --git a/src/lib_json/json_writer.cpp b/src/lib_json/json_writer.cpp
index 8e06cca2..56195dc1 100644
--- a/src/lib_json/json_writer.cpp
+++ b/src/lib_json/json_writer.cpp
@@ -178,8 +178,9 @@ static bool isAnyCharRequiredQuoting(char const* s, size_t n) {
 
   char const* const end = s + n;
   for (char const* cur = s; cur < end; ++cur) {
-    if (*cur == '\\' || *cur == '\"' || *cur < ' ' ||
-        static_cast<unsigned char>(*cur) < 0x80)
+    if (*cur == '\\' || *cur == '\"' ||
+        static_cast<unsigned char>(*cur) < ' ' ||
+        static_cast<unsigned char>(*cur) >= 0x80)
       return true;
   }
   return false;
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<div class='path'>path: <a href='/guix/log/?id=c4d769d359c67680fe55c3b71e659cd31837f3ec'>root</a>/<a href='/guix/log/gnu?id=c4d769d359c67680fe55c3b71e659cd31837f3ec'>gnu</a>/<a href='/guix/log/gnu/build?id=c4d769d359c67680fe55c3b71e659cd31837f3ec'>build</a>/<a href='/guix/log/gnu/build/svg.scm?id=c4d769d359c67680fe55c3b71e659cd31837f3ec'>svg.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/build/svg.scm?id=c4d769d359c67680fe55c3b71e659cd31837f3ec&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2018-06-01 15:21:28 +0200'>2018-06-01</span></td><td><a href='/guix/commit/gnu/build/svg.scm?id=33d8a871042e73d6b236793d02e5a8287a47ebe4'>bootloader: grub: Use 'with-extensions'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/bootloader/grub.scm (svg-&gt;png): Use 'with-extensions'.  Remove
'add-to-load-path' calls.
* gnu/build/svg.scm: Use (rsvg) and (cairo) the normal way.  Remove
'module-autoload!' calls.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-07-02 22:52:30 +0200'>2017-07-02</span></td><td><a href='/guix/commit/gnu/build/svg.scm?id=1b0f266e40aead09be95a984bd9c6cec3dff397e'>gnu: Switch guile-cairo and dependents to Guile 2.2 again.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes &lt;https://bugs.gnu.org/27551&gt;.
Reported by Leo Famulari &lt;leo@famulari.name&gt;.

This reinstates the following commits:

  e3ddb1e83 * gnu: guile-cairo: Switch to Guile 2.2.
  ae5c6ef39 * gnu: guile-gnome: Update to 2.16.5.
  0fd8013fc * gnu: guile-rsvg: Update to commit 05c6a2fd.
  66b9183c4 * gnu: guile-lib: Switch to Guile 2.2.

and adds the following changes:

* gnu/bootloader/grub.scm (svg-&gt;png): Add 'package-&gt;derivation' call for
GUILE-2.2.  Pass #:guile-for-build to 'gexp-&gt;derivation'.
* gnu/build/svg.scm (svg-&gt;png): Add 'em' and 'ex' to the 'let-values'
form to account for all the values returned by
'rsvg-handle-get-dimensions', which Guile 2.2 does not truncate.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2016-10-25 13:57:28 +0200'>2016-10-25</span></td><td><a href='/guix/commit/gnu/build/svg.scm?id=8ce84bf1f5705f5280e8bf8f150ec2e859b045ee'>svg: Autoload Guile-RSVG and Guile-Cairo.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes compilation by 'guix pull' where Guile-{RSVG,Cairo} are missing.
Reported by Efraim Flashner.

* gnu/build/svg.scm: Use 'module-autoload!' rather than 'module-use!'.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2016-10-25 01:21:28 +0200'>2016-10-25</span></td><td><a href='/guix/commit/gnu/build/svg.scm?id=ffde82c9ecf99524220e463055f4f18c8c9e7a81'>system: grub: Use Guile-{RSVG,Cairo} instead of Inkscape + ImageMagick.</a><span class='msg-avail'>...<span class='msg-tooltip'>Based on a suggestion by Andy Wingo at
&lt;https://lists.gnu.org/archive/html/guix-devel/2015-08/msg00753.html&gt;.

* gnu/build/svg.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* gnu/system/grub.scm (svg-&gt;png): Add #:width and #:height.  Rewrite to
use (gnu build svg).
(resize-image): Remove.
(grub-background-image): Adjust accordingly.
</span></span></td><td>Ludovic Courtès</td></tr>