Set environment for sage.misc.cython

[mkoeppe]

In particular libdir needs to be set correctly, using sysconfig, to ensure that it works correctly in distributions that use lib64 or multiarch installation schemes.

It may seem that nothing needs to be set because the environment variable LIBRARY_PATH is already set by sage-env. But for #30818 we wish to support also a full functional sagelib when sage-env has not been used.

See also: #30901: sage.env._get_shared_lib_filename: Fix for MULTIARCH

Depends on #31344

CC: @kiwifb @tobihan @tobiasdiez @strogdon @dimpase @orlitzky

Component: porting

Keywords: sd111

Issue created by migration from https://trac.sagemath.org/ticket/31041

[mkoeppe]

Dependencies: #31344

[mkoeppe]

comment:2

Sage development has entered the release candidate phase for 9.3. Setting a new milestone for this ticket based on a cursory review of ticket status, priority, and last modification date.

[mkoeppe]

comment:5

Another question here is whether the whole functionality of sage.misc.cython (and the IPython line/cell magic %cython) should not be split out into a separate Python distribution package.