I have windows xp, I have found some python libraries that only work for windows xp and thus if you have a mac os or linux or windows 7, you can’t download my program because it won’t work, how to make these libraries compatible with these OS, I can’t ask the creator of the libraries so I have to download the source code and modify it, and i have to make it compatible on these OS using my xp 😀 well my brother’s pc is windows 7, but I don’t have mac OS or linux (unless i can use VM)
EDIT
my application is not simple
Your question is quite broad:
1) Development and testing:
Use VMs, absolutely, they are great for testing on OS you don’t natively use, and to have a clean environment for testing (eg. test even windows stuff on a clean windows VM if you can, you might find out you’re missing some dependencies that you took for granted on your dev machine).
2) Actual library porting:
Depending on the library this may or may not be difficult. Why is this library only working on windows? does it use specific DLLs, via ctypes or swig or some other bindings. If the library is python code (not a C library), is it tied to windows python APIs?
There are many things to take into account, if using system specific APIs/libs, can they be faked on other OSs (write small abstraction over them), or does it require a lot more code. You get the gist.