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='2020-10-20 09:18:04 +0200'>2020-10-20</span></td><td><a href='/guix/commit/gnu/services/guix.scm?id=922a50993e4a33ad3758e2eae6176c3d528f26d3'>services: guix: Fix coordinator agent.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-10-08 20:21:02 +0100'>2020-10-08</span></td><td><a href='/guix/commit/gnu/services/guix.scm?id=65c9f07a1616382c00d23b83d7a42b0e72dd8df8'>services: guix: Don't use normalized codeset for Guix Data Service.</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>
<tr><td><span title='2020-10-05 18:10:48 +0100'>2020-10-05</span></td><td><a href='/guix/commit/gnu/services/guix.scm?id=c14714cbbfe9239410aec2ed73282192a64fbbdc'>services: guix: Add guix-build-coordinator-queue-builds-service-type.</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>
<tr><td><span title='2020-10-05 18:10:48 +0100'>2020-10-05</span></td><td><a href='/guix/commit/gnu/services/guix.scm?id=98ab323a246df47610652eca81494c777b50c251'>services: guix: Add guix-build-coordinator-agent-service-type.</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>
<tr><td><span title='2020-10-05 18:10:48 +0100'>2020-10-05</span></td><td><a href='/guix/commit/gnu/services/guix.scm?id=15955e9b54b8d9c1275d68a0f2b6ccbc5f6c3878'>services: guix: Add guix-build-coordinator-service-type.</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>
<tr><td><span title='2020-03-08 23:35:03 +0000'>2020-03-08</span></td><td><a href='/guix/commit/gnu/services/guix.scm?id=d8ab68b92941ca0286697b87a88254023a0b2dc9'>services: guix-data-service: Allow passing extra options.</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>
<tr><td><span title='2019-09-25 08:35:33 +0100'>2019-09-25</span></td><td><a href='/guix/commit/gnu/services/guix.scm?id=dd2a83270bccb539439b2fc831d0db04e5276f11'>services: Add the Guix Data Service.</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>