Link against libc++ on OSX

makefile

if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
  # Linking against libc++ instead of the GNU libstdc++ on OSX.
  if(APPLE)
    set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -stdlib=libc++ -lm")
    set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS}
        -stdlib=libc++ -lm")
    set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS}
        -stdlib=libc++ -lm")
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")
  else(APPLE)
    set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -lstdc++ -lm")
    set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -lstdc++ -lm")
    set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} -lstdc++ -lm")
  endif(APPLE)
endif("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")