Fix clang build for libc++

Successfully builds Qt 5.2 with:

QMAKE_CXXFLAGS_CXX11 += -std=c++11 -stdlib=libc++
QMAKE_LFLAGS_CXX11   += -stdlib=libc++ -lc++abi

against:

clang version 3.3 (tags/RELEASE_33/final)

Change-Id: I778f9410c6563e78bc77ae4c20097fa561503ba1
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>