Just because one compiler for one platform fails to support a popular C extension doesn't mean the library isn't portable.
Except that the one platform is Windows, which accounts for the vast majority of desktop PC's and laptops, and a significant chunk of servers. And the one compiler is the standard for Windows, used by the vast majority of Windows developers.
You don't have to like this, but it is the truth.
In my opinion, any software that can't compile on Windows using the native toolchain doesn't qualify as "portable". That doesn't make it bad software. It just isn't "portable" software.