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='2023-12-10 21:46:52 +0100'>2023-12-10</span></td><td><a href='/guix/commit/gnu/build/icecat-extension.scm?id=1300c316e9ab27ce042d41b3ebd6fad7e3c26efb'>gnu: ‘make-icecat-extension’ inherits package location.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2023-10-23 11:19:02 +0200'>2023-10-23</span></td><td><a href='/guix/commit/gnu/build/icecat-extension.scm?id=25043e01b6cb6696ffdc6cdedd9cdf8235bb695c'>gnu: icecat: Support Guix packaged extensions and native manifests.</a><span class='msg-avail'>...</span></td><td>Clément Lassieur</td></tr>