Edited CMakeLists.txt to support C++ 11 in XCode

Set some CMake flags to allow simbody to compile w/c++11 using Xcode. Works using OSX10.8
Ah yes there are 3 cases, but rewriting the loop in that way requires duplicating `SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")`, which is present in 2 cases (ELSEIF(APPLE) and in the ELSE). Apoorva and I tried this, and I think avoiding the duplication makes this more maintainable.