Obviously, the version check as it is now won't work once libcurl 8.0.0
is released. Use LIBCURL_VERSION_NUM to correctly check libcurl version.

Note, we can't use CURL_AT_LEAST_VERSION to check libcurl version,
because it isn't available in libcurl shipped with CentOS 6.

Fixex commit 7e62ac79 ("Add HTTP client based on libcurl").