Fix problem with awk crlf agnosticity on macOS Catalina

This changes the awk crlf agnosticity (is that a real word?) to only be used on Cygwin which is the only OS where it's needed.

This should be a general solution to the problems described in https://forum.graphviz.org/t/broken-build-due-to-commit-reversion-related-to-awk/862.

I'd appreciate if @emden could try this branch on macOS Catalina.