Make SAGE* environment variables set at configure time to stick

[kiwifb]

In #22646 the compiler configuration set at configure has been recorded for future use at runtime. This ticket aims to do the same for all the SAGE_* environment variables (where it make sense).

See also: #29507

CC: @mkoeppe @timokau

Component: build: configure

Issue created by migration from https://trac.sagemath.org/ticket/23570

[mkoeppe]

Description changed:

--- 
+++ 
@@ -1 +1,3 @@
 In #22646 the compiler configuration set at configure has been recorded for future use at runtime. This ticket aims to do the same for all the SAGE_* environment variables (where it make sense).
+
+See also: #29507

[mkoeppe]

comment:5

Setting new milestone based on a cursory review of ticket status, priority, and last modification date.