Fix build with curl < 7.26

$ grep  "define CURL_REDIR_POST_30" curl-7.2*/include/curl/curl.h

curl-7.25.0/include/curl/curl.h:#define CURL_REDIR_POST_301 1
curl-7.25.0/include/curl/curl.h:#define CURL_REDIR_POST_302 2

curl-7.26.0/include/curl/curl.h:#define CURL_REDIR_POST_301 1
curl-7.26.0/include/curl/curl.h:#define CURL_REDIR_POST_302 2
curl-7.26.0/include/curl/curl.h:#define CURL_REDIR_POST_303 4