Add DESTDIR support for zn_poly, and additional cleanup

[embray]

A bit of cleanup for zn_poly's spkg-install:

1) Implement DESTDIR support and use sage-dist-helpers where useful (#24024)

2) Remove SAGE64 support (#23733)

3) Improve Cygwin support--in particular don't install .so files to lib/ (#14208)

Depends on #25039
Depends on #23733

Component: build

Keywords: destdir zn_poly cygwin

Author: Erik Bray

Branch/Commit: a677f80

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/25085

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

cd59a43

A bit of cleanup for zn_poly's spkg-install:

[sagetrac-git]

Changed commit from ef1135e to cd59a43

[embray]

Dependencies: #25039

[embray]

comment:2

I still need to test this on Cygwin (it should be fine on OSX, but the changes on Cygwin are substantive). If it works it should also finally resolve #14208, as zn_poly.so is the last library being installed on Cygwin as a ".so".

[sagetrac-git]

Changed commit from cd59a43 to 1572604

[sagetrac-git]

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

b4ecda9	trac 23733: deprecate SAGE64 and CFLAG64
905e4d4	Stop supporting SAGE64 except in Numpy
1572604	Merge branch 'u/jdemeyer/no-sage64' into u/embray/build/destdir-zn_poly

[embray]

Changed dependencies from #25039 to #25039, #23733

[sagetrac-git]

Changed commit from 1572604 to 8168d98

[sagetrac-git]

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

8168d98

bit of a pain...