configure for old curl

Hello, @ygrek.
I'm glad your fork came to github! I thank your many bug fixes.
I saw http://forge.ocamlcore.org/forum/forum.php?forum_id=869
Does it mean that your fork has become new official now? Let me congratulate if true.

Well, I still use curl 7.19.7 that's pre-installed in Snow Leopard, and found some configuration-macros and #if are necessary to build your master with my old curl.

My old curl does not have CURL_VERSION_TLSAUTH_SRP and CURL_VERSION_NTLM_WB.
And, some other #ifdef should be replaced to #if because the HAVE_xxx macro is defined not empty but "0" if the symbol is missing.