mod_auth_cas: CAS behind proxy

[Volker Krebs]

Hello,
I've set up my Apache using mod_auth_cas, everything works fine, great 
module. Thank you for that!
But when my CAS moves behind a web-proxy I get an error
[error] [client 192.168.4.158] MOD_AUTH_CAS: connect() failed to 
https://cas.example.com/sso/login?service=http%3a%2f%2fgeier%2fvk

The Problem is, that I need a https_proxy varaible to connect to 
https://cas.example.com/. But I have no clue where to configure it in 
apache or mod_auth_cas. I've tried setting the environment variable 
https_proxy and http_proxy but apache or mod_auth_cas seem not to be 
interested.
Any Ideas ?

Thanks