The moc executable loops through headers on CPLUS_INCLUDE_PATH and stumbles
on the GCC internal _GLIBCXX_VISIBILITY macro.  Tell it to ignore it as it is
not supposed to be looking there to begin with.

Upstream report: https://bugreports.qt.io/browse/QTBUG-83160

diff --git a/src/tools/moc/main.cpp b/src/tools/moc/main.cpp
--- a/src/tools/moc/main.cpp
+++ b/src/tools/moc/main.cpp
@@ -188,6 +188,7 @@ int runMoc(int argc, char **argv)
     dummyVariadicFunctionMacro.arguments += Symbol(0, PP_IDENTIFIER, "__VA_ARGS__");
     pp.macros["__attribute__"] = dummyVariadicFunctionMacro;
     pp.macros["__declspec"] = dummyVariadicFunctionMacro;
+    pp.macros["_GLIBCXX_VISIBILITY"] = dummyVariadicFunctionMacro;
 
     QString filename;
     QString output;
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<tr><td><span title='2020-11-26 00:05:39 +0100'>2020-11-26</span></td><td><a href='/guix/commit/gnu/packages/mpi.scm?id=305eefc0627eb1d047e6fc4320d7e56897719ab8'>gnu: hwloc: Add "doc" output.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/mpi.scm (hwloc-1)[outputs]: Add "doc".
[arguments]: Add 'move-man3-pages' phase.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-11-02 14:59:34 +0100'>2020-11-02</span></td><td><a href='/guix/commit/gnu/packages/mpi.scm?id=c73496f433044a76003b33c3855bb35ecd0df87f'>gnu: mpich: Configure to use ucx.</a><span class='msg-avail'>...<span class='msg-tooltip'>Suggested by Maurice Brémond &lt;Maurice.Bremond@inria.fr&gt;.

Fixes bug when running test suites involving MPICH:
&lt;https://issues.guix.gnu.org/39588#15&gt;.

* gnu/packages/mpi.scm (mpich)[arguments]: Pass "--with-device=ch4:ucx".
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-09-24 17:53:01 +0200'>2020-09-24</span></td><td><a href='/guix/commit/gnu/packages/mpi.scm?id=215b49a8814093ef6b953b84efe30de1c3111665'>gnu: openmpi: Have 'mpirun' look for executables under $prefix.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/mpi.scm (openmpi)[arguments]: Add
"--enable-mpirun-prefix-by-default" to #:configure-flags.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-09-17 16:18:00 +0200'>2020-09-17</span></td><td><a href='/guix/commit/gnu/packages/mpi.scm?id=b96f4f0995de80032bd6e0f0b3232d426436a130'>gnu: openmpi: Update to 4.0.5.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/mpi.scm (openmpi): Update to 4.0.5.

Signed-off-by: Mathieu Othacehe &lt;othacehe@gnu.org&gt;
</span></span></td><td>Greg Hogan</td></tr>