Add cmake cross-build support
Add cmake cross-build support

Don't overwrite CPP_PLATFORM so cross-builds compile the right platform
dependent file.

basvodde/cpputest,offa/cpputest,KisImre/cpputest,offa/cpputest,Mindtribe/cpputest,cpputest/cpputest,offa/cpputest,basvodde/cpputest,offa/cpputest,PaulBussmann/cpputest,cpputest/cpputest,cpputest/cpputest,KisImre/cpputest,asgeroverby/cpputest,asgeroverby/cpputest,KisImre/cpputest,PaulBussmann/cpputest,cpputest/cpputest,PaulBussmann/cpputest,Mindtribe/cpputest,basvodde/cpputest,KisImre/cpputest,PaulBussmann/cpputest,asgeroverby/cpputest,Mindtribe/cpputest,asgeroverby/cpputest,Mindtribe/cpputest,basvodde/cpputest