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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<tr><td><span title='2021-04-12 18:42:23 +0200'>2021-04-12</span></td><td><a href='/guix/commit/gnu/tests/networking.scm?id=68c9e0a56e008f19427bd213cf5b24bdd8fe5922'>gnu: tests: Test basic funtionality of the IPFS service.</a><span class='msg-avail'>...<span class='msg-tooltip'>It is tested whether the IPFS service listens
at the gateway and API ports and whether it
is possible to upload and download a bytevector.

* gnu/tests/networking.scm
  (%ipfs-os): New variable.
  (run-ipfs-test): New procedure.
  (%test-ipfs): New system test.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Maxime Devos</td></tr>
<tr><td><span title='2020-07-11 23:36:32 +0200'>2020-07-11</span></td><td><a href='/guix/commit/gnu/tests/networking.scm?id=54461153dac6096a018e35a1d959e45b2c9dbb82'>tests: networking: Use 'net.ifnames=0' for the Open vSwitch test.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/tests/networking.scm (%openvswitch-os): Override KERNEL-ARGUMENTS.
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2020-04-10 20:20:11 +0200'>2020-04-10</span></td><td><a href='/guix/commit/gnu/tests/networking.scm?id=6720616daaf711314827c157a660339990e5cb07'>tests: openvswitch: Use the absolute file name of ovs-vsctl.</a><span class='msg-avail'>...<span class='msg-tooltip'>This fixes a regression introduced in 8b9cad01e9619f53dc5a65892ca6a09ca5de3447
where ovs-vsctl would no longer be available in PATH.

* gnu/tests/networking.scm (run-openvswitch-test): Qualify "ovs-vsctl"
invocation by ungexping the OPENVSWITCH variable instead of assuming it's
available on PATH.
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2020-03-16 14:05:52 +0100'>2020-03-16</span></td><td><a href='/guix/commit/gnu/tests/networking.scm?id=ef0f5ff2a74e5e4dc49dfdd0ba6509a91281ddd5'>tests: dhcpd: Adjust network interface name.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is a followup to 8e53fe2b91d2776bc1529e7b34967c8f1d9edc32.

* gnu/tests/networking.scm (dhcpd-v4-configuration)
(%dhcpd-os): Use "ens3" instead of "eth0".
</span></span></td><td>Ludovic Courtès</td></tr>