CMake >= 3.27 exports the CXX std to moc. This is a workaround for
a moc parsing bug that happens parsing headers from libc++ < 14 when
the CXX std > 17.

usr/include/c++/v1/concept:318:1: error: Parse error at "::"

Moc should theoretically be able to parse this. [1] Not exactly sure
why it's failing. Upstream bug report [2]

[1] https://code.qt.io/cgit/qt/qtbase.git/tree/src/tools/moc/moc.cpp?h=6.6.0#n631
[2] https://bugreports.qt.io/browse/QTBUG-117765

Index: Source/cmQtAutoGenInitializer.cxx
--- Source/cmQtAutoGenInitializer.cxx.orig
+++ Source/cmQtAutoGenInitializer.cxx
@@ -1809,6 +1809,10 @@ bool cmQtAutoGenInitializer::SetupWriteAutogenInfo()
       if (this->Moc.PredefsCmd.size() >= 3) {
         this->Moc.PredefsCmd.insert(this->Moc.PredefsCmd.begin() + 1,
                                     CompileOptionValue);
+#if (defined(__FreeBSD__) && __FreeBSD_version < 1302000) || defined(__OpenBSD__)
+        this->Moc.PredefsCmd.insert(this->Moc.PredefsCmd.begin() + 1,
+                                    "-D_LIBCPP_HAS_NO_CONCEPTS");
+#endif
       }
     }
     info.SetArray("MOC_PREDEFS_CMD", this->Moc.PredefsCmd);