[make] Support windows in makefile

[tobil4sk]

On windows, the makefile fails because pwd returns a unix style path. This changes the makefile to use windows paths if windows is detected. This allows a successful build on windows.

[chetmurthy]

I deleted my previous comment: your Windows changes worked (I made a clerical error).

I made a mistake when I created this repo, somehow the default branch got set to "main" even though I always use "master". I changed that just now, just thought I should let you know.

[chetmurthy]

FYI, I changed "main" to "main-DO-NOT-USE" so it would be obvious.

[chetmurthy]

Ugh, the opam-ci windows build is .... pretty opaque, and it seems like in fact the windows build failed after all. I think it's b/c revdeps failed (b/c most of the pa_ppx_* packages are not gonna work on windows). Sigh. Let's see where things shake out in the morning. In any case, I'll try to push thru your camlp5-on-Windows fixes: maybe over time I can fixup enough of the pa_ppx packages that the Windows CI builds start working.

[tobil4sk]

Many thanks! Let's see how it goes.