Fix Qt 4.8.4 build when using Xcode 4.6's clang.

Backports https://bugreports.qt-project.org/browse/QTBUG-29373 .

`./scripts/macosx-build-dependencies.sh  -c` finishes successfully for me with this change. Building without `-c` fails somewhere in boost. You probably want to turn on `-c` by default on mac, or mention on the README that it's required at least with recent Xcode versions.