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=3403a4201057a9f7b02d332029d0c4f2ba158376'>root</a>/<a href='/guix/log/tests?id=3403a4201057a9f7b02d332029d0c4f2ba158376'>tests</a>/<a href='/guix/log/tests/ui.scm?id=3403a4201057a9f7b02d332029d0c4f2ba158376'>ui.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/ui.scm?id=3403a4201057a9f7b02d332029d0c4f2ba158376&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-09-19 23:24:04 +0200'>2019-09-19</span></td><td><a href='/guix/commit/tests/ui.scm?id=d2cdef65605b9e14bfa02c3bf1612ab6b62f4a89'>ui: 'relevance' connects regexps with a logical and.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes &lt;https://bugs.gnu.org/36763&gt;.
Previously, the logical and connecting the regexps did not output the expected
results (introduced in 8874faaaac665100a095ef25e39c9a389f5a397f).

* guix/ui.scm (relevance)
[score]: Change its arguments.
[regexp-&gt;score]: New procedure.
* tests/ui.scm ("package-relevance"): Add test.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>zimoun</td></tr>
<tr><td><span title='2019-06-27 11:14:40 +0200'>2019-06-27</span></td><td><a href='/guix/commit/tests/ui.scm?id=8874faaaac665100a095ef25e39c9a389f5a397f'>ui: 'relevance' considers regexps connected with a logical and.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/ui.scm (relevance)[score]: Change to return 0 when one of REGEXPS
doesn't match.
* tests/ui.scm ("package-relevance"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-05-10 22:05:21 +0200'>2017-05-10</span></td><td><a href='/guix/commit/tests/ui.scm?id=30d2397f73d6fa9e77eed70ec6df8afd3b9f5f5e'>ui: 'string-&gt;duration' correctly handles hours.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/ui.scm (string-&gt;duration): Add missing '=&gt;' for hours.
* tests/ui.scm ("duration, 2 hours"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-03-08 23:42:53 +0100'>2017-03-08</span></td><td><a href='/guix/commit/tests/ui.scm?id=1062063a65036f32a061c54866d33ffe079ec44e'>ui: Don't use '%default-port-encoding' to set the encoding of string ports.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/ui.scm (right-arrow): Call 'set-port-encoding!' to set PORT's
encoding; remove use of 'with-fluids'.  This is for compatibility with
Guile 2.2 where the encoding of string ports is not influenced by
%DEFAULT-PORT-ENCODING.
* tests/ui.scm ("show-manifest-transaction"): Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>