Here is a list (in-progress) of the things needs to get Sage to build (and pass most tests) on Cygwin:

• #8257: gd needs a better version of expr

• #7319: gdmodule fails

• NTL should not build a static library

• Pynac needs to use a function pointer table.

• #8642: port detection for the notebook does not work in Cygwin