Support for compiling with Clang c++11

Allows compiling with Clang on OS X 10.8.2 with the following configuration:

cmake -DBUILD_opencv_java=OFF -DBUILD_opencv_softcascade=OFF -DCMAKE_CXX_FLAGS="-std=c++11 -stdlib=libc++" -DCMAKE_SHARED_LINKER_FLAGS="-stdlib=libc++" -DCMAKE_EXE_LINKER_FLAGS="-stdlib=libc++" ..