The file InfinityWindoid.π is a THINK C 5.0 project file.
In order to compile the Windoid WDEF under THINK C 6.0, simply open this project file with THINK C 6.0 and allow it to convert the file for use with the newer version. I have tested this procedure and it works fine, so I found it unnecessary to include project files for both version of THINK C.
Note, however, that doing this will cause the project file to be unusable with THINK C 5 in the future. (If this is important, simply duplicate the project file before opening it in THINK C 6 so that you have an untouched version.)