Replace existing environment variable based revision number support by a file-based method
- Generate a special header file in the build output folder with the current revision number
- Include the new header file from internal_version.h when a specific define is set
- Update create_project to define SCUMMVM_INTERNAL_REVISION as needed and add the build output folder to the include path
- Remove support for git-svn clones in the revision script (not useful anymore after the switch to git)
