http.c: don't use curl_easy_strerror prior to curl-7.12.0

Reverts be22d92e (http: avoid empty error messages for some curl
errors, 2011-09-05) on platforms with older versions of libcURL
where the function is not available.
Signed-off-by: NJoachim Schmitz <jojo@schmitz-digital.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

http.c

@@ -806,10 +806,12 @@ static int http_request(const char *url, void *result, int target, int options)
 				ret = HTTP_REAUTH;
 			}
 		} else {
+#if LIBCURL_VERSION_NUM >= 0x070c00
 			if (!curl_errorstr[0])
 				strlcpy(curl_errorstr,
 					curl_easy_strerror(results.curl_result),
 					sizeof(curl_errorstr));
+#endif
 			ret = HTTP_ERROR;
 		}
 	} else {