RFC: writable private scratch XDG_CACHE_HOME in build enviroment?

[Andy Wingo]

Hi,

Guile will try to automatically compile .scm files and cache them in
XDG_CACHE_HOME.  When building Guile itself, Guile sets XDG_CACHE_HOME
to ${top_builddir}/cache.  What if we would set this ourselves for all
packages?  That way all packages could benefit from a scratch location
when building that wouldn't propagate to the outputs.

In Fibers I have some tests that I assume get compiled.  If they're not
compiled, they go quite slow (260s vs 5s).  Alternately I could add a
XDG_CACHE_HOME setting in Fibers.  Which should we do?

Regards,

Andy