libetpan on macOS 10.5–10.6 and CFNetwork

[barracuda156]

@dinhvh Could you please say whether linking to CFNetwork is something essential for libetpan? That framework appears only in 10.8 (though at least some functions exist in 10.6+), however the commit 5f535b0 that introduced references to related code still has 10.5 as the SDK root.

AFAICT, I also had libetpan building on 10.6 for ppc, though I did not test its functionality (away from PowerPC hardware at the moment).

We would like to have it working on 10.5+, to the extent possible.

P. S. For the context, I bumped into this issue with nmail, which uses libetpan in the codebase. See: d99kris/nmail#153