Fix Python 2 build on Cygwin

[embray]

This commit broke the build of Python 2 on Cygwin by moving the definition of the PYTHON_VERSION variable used for the Cygwin install as well as OSX.

I didn't notice before since I was just doing incremental builds.

Component: porting: Cygwin

Author: Erik Bray

Branch/Commit: 609393a

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/23059

[embray]

Branch: u/embray/cygwin/ticket-23059

[embray]

Author: Erik Bray

[sagetrac-git]

Commit: 609393a

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

609393a

Roll back accidental change from dc46ea2dfa97be79d7ef8ddbe9130d4c717acc3d that broke build of Python 2 on Cygwin.

[tscrim]

comment:3

LGTM.

[tscrim]

Reviewer: Travis Scrimshaw

[jhpalmieri]

comment:4

Sorry, that was a silly mistake on my part.

[embray]

comment:5

No problem!