Winelib is a development toolkit which allows you to compile your Windows applications on Unix.
Most of Winelib code consists of the Win32 API implementation. Fortunately this part is 100 percent shared with Wine. The remainder consists of Windows compatible headers and tools like the resource compiler (and even these are used when compiling Wine).
Thanks to the above, Winelib supports most C and C++ source code, resource and message files, and can generate graphical or console applications as well as dynamic libraries.
There is no need to do that since 15 years ago Icculus already made native Linux version for games on that engine. It's just question of time when he'll be allowed to release source code and manage to merge it into released codebase.
Also Winelib isn't something that magically make Windows-only software compile on Linux. It's just helper library that may make it easier, but it's usually wrong way to do things since it's not going to be run better than normal Wine.
There most likely some legal issues (like he probably had to sign some sort of contract that protected the source or some shit), but luckily both parties involved seem to want to open the Linux port. So it should really be an issue. And I think he mostly ends up using SDL rather than middleware that would cause issues.
Somewhat hoping it's an oversight and not some legal hangup.