Hopefully this will work on Win10 as well... Only time will tell!
Here's what's changed:
1. Added a platform_winrt.cpp file. It's based on platform_windows.c but has some WinRT-specific changes.
2. Changed physfs_platforms.h to define PHYSFS_PLATFORM_WINRT when running on WinRT. PHYSFS_PLATFORM_WINDOWS should also be defined, as WinRT behaves a lot like "normal" Windows and this flag is tested against in multiple parts of the PhysFS source code.
3. Changed platform_windows.c to not be used when PHYSFS_PLATFORM_WINRT is defined.
4. In order to work on Windows ARM devices, I had to change in physfs_internal.h, to include _M_ARM as a flag for running on ARM as "ARM" and "__arm__" aren't defined when running on a Windows ARM device.
What I've tested:
* Setting up a "sane folder configuration"
* Reading and writing files
* Mounting zip files
* Using seek & tell
* Checking file length
All of these work fine. I couldn't get the actual test software to run on WinRT, but it seems like eveyrthing's working. I've tested these things on my Windows 8.1 computer, as well as my Lumia 920 which is currently running Windows 8.1.