Add DESTDIR support for sympow

[embray]

Implements #24024 and #23733 for sympow.

This one is a bit of an odd duck, and I left a lot of it alone. But there were still a few areas where it could benefit from a bit of simplification.

Depends on #25039
Depends on #23733

CC: @slel

Component: build

Keywords: destdir sympow

Author: Erik Bray

Branch/Commit: ed0de78

Reviewer: Julian Rüth

Issue created by migration from https://trac.sagemath.org/ticket/25099

[embray]

Changed dependencies from #25039 to #25039, #23733

[sagetrac-git]

Changed commit from 8c9a257 to 1619079

[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
1619079	Merge branch 'u/jdemeyer/no-sage64' into u/embray/build/destdir-sympow

[saraedum]

Reviewer: Julian Rüth

[saraedum]

comment:4

embray: What's the status of sympow? Are we essentially the upstream maintainers of that package?

[embray]

comment:5

I have no idea.