How to set environment variables for X session? [Was Re: Any idea why starting evolution...]

[Chris G]

What's the proper/official way to set environment variables for an X
session?  I.e. how do I run a command and set some variables early in
the X session startup so that they will be available to any/all programs
in the X session?

Setting them in .xprofile isn't early enough, that only sets variables
for programs run in terminal windows far as I can tell.

-- 
Chris Green