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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<div class='path'>path: <a href='/guix/log/?id=b1f31d4e47730ba6acda1118d8816c97134c6076'>root</a>/<a href='/guix/log/build-aux?id=b1f31d4e47730ba6acda1118d8816c97134c6076'>build-aux</a>/<a href='/guix/log/build-aux/pre-inst-env.in?id=b1f31d4e47730ba6acda1118d8816c97134c6076'>pre-inst-env.in</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/build-aux/pre-inst-env.in?id=b1f31d4e47730ba6acda1118d8816c97134c6076&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-08-29 16:24:17 +0200'>2020-08-29</span></td><td><a href='/guix/commit/build-aux/pre-inst-env.in?id=d7965b58d25c9c7648ba5682223f32d759fdffda'>build: Remove references to the 'nix-hash' program.</a><span class='msg-avail'>...<span class='msg-tooltip'>* configure.ac: Remove check for 'nix-hash'.
* tests/base32.scm (%nix-hash, %have-nix-hash?): Remove.
("sha256 &amp; bytevector-&gt;nix-base32-string"): Remove test.
* build-aux/pre-inst-env.in: Do not set 'NIX_HASH' environment
variable.
</span></span></td><td>Ludovic Courtès</td></tr>