Made TargetFramework constant.
Made TargetFramework constant.

Should preserve compatibility with older
Mono versions.

Closes #1123

cake-build/cake,Julien-Mialon/cake,Sam13/cake,phenixdotnet/cake,phrusher/cake,vlesierse/cake,thomaslevesque/cake,phrusher/cake,gep13/cake,michael-wolfenden/cake,ferventcoder/cake,DixonD-git/cake,Julien-Mialon/cake,vlesierse/cake,mholo65/cake,adamhathcock/cake,RehanSaeed/cake,thomaslevesque/cake,robgha01/cake,devlead/cake,Sam13/cake,patriksvensson/cake,daveaglick/cake,patriksvensson/cake,phenixdotnet/cake,adamhathcock/cake,cake-build/cake,mholo65/cake,ferventcoder/cake,michael-wolfenden/cake,devlead/cake,gep13/cake,daveaglick/cake,robgha01/cake,RehanSaeed/cake