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
Oh, I see -- I didn't catch that it was adding *two* flags in the APPLE case. I do think that's confusing. I think it would be more clear if the three cases could be made more obvious. How about a structure like this:
```
if (APPLE)
   if(XCODE) do xcode stuff
   else do non-xcode apple stuff
else() # not APPLE
  do non-apple stuff
endif()
```
I don't think it would be a problem if the same flag appeared in more than one of those cases.