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);
 }
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<div class='path'>path: <a href='/guix/log/?id=da381e233453b746b77b899b0584a13c41202e4c'>root</a>/<a href='/guix/log/tests?id=da381e233453b746b77b899b0584a13c41202e4c'>tests</a>/<a href='/guix/log/tests/gexp.scm?id=da381e233453b746b77b899b0584a13c41202e4c'>gexp.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/tests/gexp.scm?id=da381e233453b746b77b899b0584a13c41202e4c&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2023-12-22 00:31:40 +0100'>2023-12-22</span></td><td><a href='/guix/commit/tests/gexp.scm?id=11a454f9dae84cc00b977d164dae764454ecb11d'>gexp: #:references-graphs accepts and honors &lt;gexp-input&gt; records.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2023-12-22 00:31:40 +0100'>2023-12-22</span></td><td><a href='/guix/commit/tests/gexp.scm?id=d9190abbd20f15ea5b55abdd51e1376f05055850'>gexp: Add compiler for &lt;gexp-input&gt;.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2023-11-25 16:29:25 +0100'>2023-11-25</span></td><td><a href='/guix/commit/tests/gexp.scm?id=9b48cf8cdd5718bc35828b3c4c9abe84122dad96'>tests: Import (guix build syscalls) when (guix build store-copy) is used.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-11-11 23:17:42 +0100'>2022-11-11</span></td><td><a href='/guix/commit/tests/gexp.scm?id=19206eee69e8c22d63104af1b7f1f815969bff7f'>grafts: Run with a UTF-8 locale.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-10-22 01:46:55 +0200'>2022-10-22</span></td><td><a href='/guix/commit/tests/gexp.scm?id=a3619079f95213c4f983e69210ed12b38fd31022'>Remove now unnecessary uses of (guix grafts).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-05-25 09:28:43 +0200'>2022-05-25</span></td><td><a href='/guix/commit/tests/gexp.scm?id=d276a7dd6162d101b7790cc8ea902abfec1eafcb'>tests: Fix wrong target triplets.</a><span class='msg-avail'>...</span></td><td>Josselin Poiret</td></tr>
<tr><td><span title='2022-05-01 21:30:34 +0200'>2022-05-01</span></td><td><a href='/guix/commit/tests/gexp.scm?id=774f8804bafbf42a65eca492d1395da57deeb467'>gexp: Add 'references-file'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-03-13 23:20:07 +0100'>2022-03-13</span></td><td><a href='/guix/commit/tests/gexp.scm?id=2e5c3d91fea5b583c541d70b4c318c2f81554d43'>gexp: Correctly handle #$output in 'gexp-&gt;approximate-sexp'.</a><span class='msg-avail'>...</span></td><td>Maxime Devos</td></tr>
<tr><td><span title='2022-03-13 23:18:14 +0100'>2022-03-13</span></td><td><a href='/guix/commit/tests/gexp.scm?id=5aec62ee0f69d691c1c1e322029463beb8bfc3cd'>gexp: Correctly handle unquoting S-exp objects.</a><span class='msg-avail'>...</span></td><td>Maxime Devos</td></tr>
<tr><td><span title='2022-02-16 16:49:26 +0100'>2022-02-16</span></td><td><a href='/guix/commit/tests/gexp.scm?id=ca155a20aea25003b03ef5e0420c77e416d5f425'>gexp: Preserve source location for #~ and #$ read extensions.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-02-15 09:33:59 +0100'>2022-02-15</span></td><td><a href='/guix/commit/tests/gexp.scm?id=7f6dd3be3dceb0fda15fd02c9165614b2626813e'>gexp: 'computed-file' always uses a native Guile.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-02-11 23:51:39 +0100'>2022-02-11</span></td><td><a href='/guix/commit/tests/gexp.scm?id=bc1ad69605b6c3e2744694758d63dfc216107a82'>tests: Pass #:guile to 'computed-file' &amp; co.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>