How to compile apps with g77 on OS X Lion?
g77 is not part of Xcode. You may have a look here http://hpc.sourceforge.net/ which provides a version compatible with Lion and XCode 4.x. 


  On LION, with XCode 4.3 you will need to download the command-line tools as an additional step. You will find the option to download the command-line tools in XCode 4.3's Preferences