http: don't hardcode the value of CURL_SOCKOPT_OK

Use the new git-curl-compat.h header to define CURL_SOCKOPT_OK to its
known value if we're on an older curl version that doesn't have it. It
was hardcoded in http.c in a15d069 (http: enable keepalive on TCP
sockets, 2013-10-12).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-curl-compat.h

@@ -20,10 +20,21 @@
  * GIT_CURL_HAVE_X. If multiple similar symbols with the same prefix
  * were defined in the same version we pick one and check for that name.
  *
+ * We may also define a missing CURL_* symbol to its known value, if
+ * doing so is sufficient to add support for it to older versions that
+ * don't have it.
+ *
  * Keep any symbols in date order of when their support was
  * introduced, oldest first, in the official version of cURL library.
  */
 
+/**
+ * CURL_SOCKOPT_OK was added in 7.21.5, released in April 2011.
+ */
+#if LIBCURL_VERSION_NUM < 0x071505
+#define CURL_SOCKOPT_OK 0
+#endif
+
 /**
  * CURLOPT_TCP_KEEPALIVE was added in 7.25.0, released in March 2012.
  */

http.c

@@ -537,7 +537,7 @@ static int sockopt_callback(void *client, curl_socket_t fd, curlsocktype type)
 	if (rc < 0)
 		warning_errno("unable to set SO_KEEPALIVE on socket");
 
-	return 0; /* CURL_SOCKOPT_OK only exists since curl 7.21.5 */
+	return CURL_SOCKOPT_OK;
 }
 
 static void set_curl_keepalive(CURL *c)