VS2010 Warnings about _ALLOW_KEYWORD_MACROS

I think juce_PlatformDefs.h should read:

 #if defined (_MSC_VER) && _MSC_VER > 1600
  #define _ALLOW_KEYWORD_MACROS 1 // (to stop VC2012 complaining)
 #endif

Oh, ok. I assumed that macro was unused in earlier versions. Will update that.