re-enable caching of cython dependencies during build

[craigcitro]

We decided to temporarily disable the caching of the cython dependencies during the build, simply because it was causing so much grief. However, this should be re-enabled once someone takes the time to sit down and work out the last kinks. In particular, removing files from the sage tree and rebuilding tends to cause exceptions.

See $SAGE_ROOT/devel/sage/setup.py for some comments, and to play with this.

Component: build

Reviewer: Jeroen Demeyer

Issue created by migration from https://trac.sagemath.org/ticket/4651

[jdemeyer]

comment:1

Dependency checking is in upstream Cython now.

[jdemeyer]

Reviewer: Jeroen Demeyer