Fix cygwin build configuration and code switches

[theurich]

Start the review process of changes provided by @robin-deltares.

See the associated issue: #136

This PR needs very careful review. A lot of source files have been touched. Best to work on this when having access to a Cygwin system.

[DWesl]

You could use GitHub Actions to get a Cygwin installation, I have an example here: https://github.com/DWesl/esmf/tree/esmf-cygwin-compile
It takes two hours to compile and one to run the checks, roughly the same as on a laptop. It is a bit more difficult to work out which flags guard various definitions (for example, math.h defines M_PI if you pass -D_BSD_SOURCE on the command line).

The doc build isn't working at the moment, and I'm not sure how to fix that.