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 <tgc@statsbiblioteket.dk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

@@ -297,7 +297,9 @@ static CURL *get_curl_handle(void)
 
 	if (curl_http_proxy) {
 		curl_easy_setopt(result, CURLOPT_PROXY, curl_http_proxy);
+#if LIBCURL_VERSION_NUM >= 0x070a07
 		curl_easy_setopt(result, CURLOPT_PROXYAUTH, CURLAUTH_ANY);
+#endif
 	}
 
 	return result;