http: support curl < 7.10.7
http: support curl < 7.10.7

Commit dd61399 introduced support for http proxies that require
authentication but it relies on the CURL_PROXYAUTH option which was
added in curl 7.10.7.
This makes sure proxy authentication is only enabled if libcurl can
support it.

Signed-off-by: Tom G. Christensen <1c1659c8b732dbc7a2ecc185c98f9f178154b466@statsbiblioteket.dk>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git