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='2020-03-14 12:02:52 +0100'>2020-03-14</span></td><td><a href='/guix/commit/gnu/packages/pcre.scm?id=704f1b53c77f3550bc7b3e86aa7237db2fc1594e'>gnu: pcre: Update to 8.44.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/pcre.scm (pcre): Update to 8.44.
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2019-12-11 23:21:55 +0100'>2019-12-11</span></td><td><a href='/guix/commit/gnu/packages/pcre.scm?id=f6943db7d39a060742b820e2f2f215b0bdeca690'>gnu: pcre2: Do not build static libraries.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/pcre.scm (pcre2)[arguments]: Add "--disable-static" to
 #:configure-flags.
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2019-12-11 23:21:55 +0100'>2019-12-11</span></td><td><a href='/guix/commit/gnu/packages/pcre.scm?id=eccd2c14d6b42142e85ed517e8cfa81a73b6bd88'>gnu: pcre2: Update to 10.34.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/patches/pcre2-fix-jit_match-crash.patch: Delete file.
* gnu/local.mk (dist_patch_DATA): Adjust accordingly.
* gnu/packages/pcre.scm (pcre2): Update to 10.34.
[source](patches): Remove.
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2019-08-13 20:42:06 +0200'>2019-08-13</span></td><td><a href='/guix/commit/gnu/packages/pcre.scm?id=462ad9f56b9665b8d22960acee73ad91f1052c9c'>Merge branch 'master' into core-updates</a></td><td>Marius Bakke</td></tr>
<tr><td><span title='2019-08-04 13:49:45 +0200'>2019-08-04</span></td><td><a href='/guix/commit/gnu/packages/pcre.scm?id=a33925d122c99eafb6e1e53ab1c122d7245d8a78'>gnu: Remove pcre2/fixed.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/pcre.scm (pcre2/fixed): Remove variable.
(pcre2)[replacement]: Remove.
[source](patches): New field.
* gnu/packages/php.scm (php)[inputs]: Change PCRE2/FIXED to PCRE2.
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2019-08-02 01:49:47 +0200'>2019-08-02</span></td><td><a href='/guix/commit/gnu/packages/pcre.scm?id=951fd8572af1bd00064d5c0ae59c2927ec1b6ba5'>gnu: pcre@2: Fix run-time crash [security fix].</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/pcre.scm (pcre2)[replacement]: New field.
(pcre2/fixed): New public variable.
* packages/patches/pcre2-fix-jit_match-crash.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2019-05-25 00:25:15 +0200'>2019-05-25</span></td><td><a href='/guix/commit/gnu/packages/pcre.scm?id=57df83e07d4b5e78d9a54c1a88d05b4a9ed65714'>Merge branch 'staging' into core-updates</a></td><td>Marius Bakke</td></tr>
<tr><td><span title='2019-05-04 16:53:49 +0200'>2019-05-04</span></td><td><a href='/guix/commit/gnu/packages/pcre.scm?id=9adae0a0176caf38519f3b3e30b7855fbc70c548'>gnu: pcre2: Update to 10.33.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/pcre.scm (pcre2): Update to 10.33.
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2019-03-04 22:58:59 +0100'>2019-03-04</span></td><td><a href='/guix/commit/gnu/packages/pcre.scm?id=dd7ce8643a28f5d633c5f3124de6be897cd5065f'>gnu: pcre: Update to 8.43.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/pcre.scm (pcre): Update to 8.43.
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2019-02-07 22:10:17 +0100'>2019-02-07</span></td><td><a href='/guix/commit/gnu/packages/pcre.scm?id=d9464ca53ab66a92d1d223738227a23e8ad1b86b'>gnu: pcre2: Update to 10.32.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/pcre.scm (pcre2): Update to 10.32.
</span></span></td><td>Marius Bakke</td></tr>