add support for windows

[oreparaz]

@0adb reported in #3 that p256 doesn't compile cleanly in Windows 10 using MinGW. I don't have access right now to a Windows box so maybe we can debug with them. @0adb: could you please check if setting this environment variable in powershell helps compiling in windows?

$Env:CC="gcc"

Also: since I essentially ripped off the project structure from @charlesnicholson 's embedded libraries, and they compile in Windows, maybe we can just use their make recipe (it is using MSVC instaead of MinGW, tho): https://github.com/charlesnicholson/nanocobs/blob/main/make-win.bat

[charlesnicholson]

:-D BTW I'm guessing you saw this but just to say it explicitly: my make-win.bat thing is kinda janky and doesn't do any sort of incremental rebuild. If I were developing on windows as a first-class thing I'd probably do something more robust with a cross-platform build system (CMake, Meson, etc).

[oreparaz]

that's a great idea, ty @charlesnicholson . def agree something like CMake would be cleaner. it's a bit more complex but we have a good example in nanoarq: https://github.com/charlesnicholson/nanoarq/blob/master/CMakeLists.txt . Will add as future work.

@0adb if you're using MinGW then I suppose the provided Makefile should work (perhaps with minor tweaking like adjusting your PATH) and hopefully that can get you started. Sorry I don't have a windows machine to try this on! You don't need to port to CMake since that's a larger endeavor.

[charlesnicholson]

(BTW, I don't know if or how literally you want to follow that CMakeLists.txt, but if I were writing it from scratch I'd probably submodule a tag of a test framework instead of using ExternalProject_Add.)