Fix a string comparison in C++ 11.

Taken from Debian:
https://sources.debian.org/src/dbus-c++/0.9.0-8.1/debian/patches/06_fix_gcc-7_ftbfs.patch/

--- a/src/pipe.cpp
+++ b/src/pipe.cpp
@@ -83,5 +83,5 @@ ssize_t Pipe::read(void *buffer, unsigne
 void Pipe::signal()
 {
   // TODO: ignoring return of read/write generates warning; maybe relevant for eventloop work...
-  ::write(_fd_write, '\0', 1);
+  ::write(_fd_write, "\0", 1);
 }
-static/cgit.png' alt='cgit logo'/></a></td>
<td class='main'><a href='/'>index</a> : <a href='/guix/'>guix</a></td><td class='form'><form method='get'>
<input type='hidden' name='id' value='e7971cb430d45f9ad8fcc0fe5bd88e1b863c5047'/><select name='h' onchange='this.form.submit();'>
<option value='koszko' selected='selected'>koszko</option>
<option value='koszko-scripts'>koszko-scripts</option>
</select> <input type='submit' value='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=e7971cb430d45f9ad8fcc0fe5bd88e1b863c5047'>refs</a><a class='active' href='/guix/log/gnu/tests.scm'>log</a><a href='/guix/tree/gnu/tests.scm?id=e7971cb430d45f9ad8fcc0fe5bd88e1b863c5047'>tree</a><a href='/guix/commit/gnu/tests.scm?id=e7971cb430d45f9ad8fcc0fe5bd88e1b863c5047'>commit</a><a href='/guix/diff/gnu/tests.scm?id=e7971cb430d45f9ad8fcc0fe5bd88e1b863c5047'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/tests.scm'>
<input type='hidden' name='id' value='e7971cb430d45f9ad8fcc0fe5bd88e1b863c5047'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=e7971cb430d45f9ad8fcc0fe5bd88e1b863c5047'>root</a>/<a href='/guix/log/gnu?id=e7971cb430d45f9ad8fcc0fe5bd88e1b863c5047'>gnu</a>/<a href='/guix/log/gnu/tests.scm?id=e7971cb430d45f9ad8fcc0fe5bd88e1b863c5047'>tests.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/tests.scm?id=e7971cb430d45f9ad8fcc0fe5bd88e1b863c5047&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-04-12 18:42:22 +0200'>2021-04-12</span></td><td><a href='/guix/commit/gnu/tests.scm?id=3332f4365bf329c361d650c70e53b8a74ad923bd'>tests: Support package extensions in the backdoor REPL.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/tests.scm
  (&lt;marionette-configuration&gt;): Add 'extensions' field.
  (marionette-shepherd-service): Honour the field.
  (with-import-modules-and-extensions): Define a combination
  of 'with-import-modules' and 'with-extensions'.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Maxime Devos</td></tr>
<tr><td><span title='2021-02-19 20:10:08 +0100'>2021-02-19</span></td><td><a href='/guix/commit/gnu/tests.scm?id=f57b7cea748c1e8284373811c13398d9f69aec89'>tests: Export %simple-os.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/tests.scm (%simple-os): Export it.
</span></span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-07-25 19:11:36 +0200'>2020-07-25</span></td><td><a href='/guix/commit/gnu/tests.scm?id=a5e2fc73760a2ae023f2e56bdbf8025971f90e64'>utils: Move &lt;location&gt; and '&amp;error-location' to (guix diagnostics).</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/utils.scm (&lt;location&gt;, source-properties-&gt;location)
(location-&gt;source-properties, &amp;error-location): Move to...
* guix/diagnostics.scm: ... here.
* gnu.scm: Adjust imports accordingly.
* gnu/machine.scm: Likewise.
* gnu/system.scm: Likewise.
* gnu/tests.scm: Likewise.
* guix/inferior.scm: Likewise.
* tests/channels.scm: Likewise.
* tests/packages.scm: Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-03-05 16:33:37 +0100'>2020-03-05</span></td><td><a href='/guix/commit/gnu/tests.scm?id=c0b726c2112e0e670dc134583b376541c0a3789a'>tests: Add &lt;system-test&gt; gexp compiler.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/tests.scm (compile-system-test): New gexp compiler.
</span></span></td><td>Ludovic Courtès</td></tr>