Makefiles: fix KRML_HOME to work with Cygwin

[tahina-pro]

Some Makefiles were defining KRML_HOME with Cygwin paths. This PR fixes them to native Windows paths.

[msprotz]

CURDIR is good but only supported by GNU Make, so if this Makefile doesn't do it already, I would suggest aborting in case someone is running e.g. BSD make

[tahina-pro]

Thanks Jonathan for the heads up. What about $(shell pwd), as is currently done in master? Do any BSD variants of make support it? If so, then I'd rather switch back to $(shell pwd)

(For the record, with this PR as it is now, ./everest make and ./everest test successfully work on Windows+Cygwin with official opam 2.3 (and OCaml 5.2.1): https://github.com/project-everest/everest/actions/runs/12239757622 .)

[msprotz]

I'm not sure $(shell pwd) works if you use make -C from a different directory. Can you confirm?

In general, I am against attempting to write portable Makefiles. It results in more complicated, less readable, less maintainable Makefiles, and we already assume GNU Make in other parts of Everest. Plus, most of our makefiles have implicit dependencies on GNU Make. So, your earlier solution was great, and I'm in support of it, just error out if you're running with GNU Make.

[tahina-pro]

Thanks Jonathan!
Yes, I can confirm $(shell pwd) works well with make -C dir. (It is the case right now with Karamel master and ./everest make)

I didn't find any reliable way to check for GNU make other than renaming the Makefile into GNUmakefile, per https://www.gnu.org/software/make/manual/html_node/Makefile-Names.html :

The first name checked, GNUmakefile, is not recommended for most makefiles. You should use this name if you have a makefile that is specific to GNU make, and will not be understood by other versions of make. Other make programs look for makefile and Makefile, but not GNUmakefile.

Alternatively, the check in the HACL* Makefile at https://github.com/hacl-star/hacl-star/blob/b228390cd45329996e80380135d17410bc23e4a7/Makefile#L46
can rule out OSX Make. Would that be enough?

[msprotz]

I think it's good as it is then. No need to waste your precious time with any of these details, you make no more assumptions than we already do in the existing build and other parts of Everest. Thank you for double-checking!

[tahina-pro]

Thanks a lot Jonathan for merging!