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
The indentation doesn't look right here. I suspect you may have inserted tabs rather than spaces. If so, you should always set your editor to substitute n spaces (usually 4 is good) when you hit tab (all editors have that option). Using actual tabs always causes problems.