modify custom variables

Hi,

When creating postgis extension, a call to DefineCustomStringVariable is
made to define postgis.backend.

An assign hook allows to change the library used by spatial functions
whith a:

SET postgis.backend = 'sfcgal'; -- or 'geos'

Everything work fine execept when upgrading postgis.

If I understand the problem correctly, on upgrate (ALTER EXTENSION), the
old postgis library is already loaded, the old spatial functions are
replaced by new ones pointing to the new postgis library, a call to
DefineCustomStringVariable is made and:

ERROR: attempt to redefine parameter "postgis.backend"

I need to redefine the assign hook such that is calls the new library
backend and not the old one.

Is there a way to remove/modify custom variables ? I looked in guc.h but
couldn't find anything.

Thanks,

V.

PS: I'm working on http://trac.osgeo.org/postgis/ticket/2382.