[BUILD] Build break with old curl, macro CURL_VERSION_BITS unknown

[marcalff]

When compiling code that depends on CURL, like the OTLP HTTP exporter, the build fails for old curl.

The issue is that file:
ext/src/http/client/curl/http_operation_curl.cc
contains uses of the CURL macro CURL_VERSION_BITS.

This macro was introduced only in CURL 7.43.0, so building with older curl breaks.

Fix:

• Keep using CURL_VERSION_BITS for readability
• define the macro if missing.

Curl commit that introduced this macro:

commit 39b9bf60d13ff7cb62a6a031c210d27a32113c4f
Author: Daniel Stenberg <daniel@haxx.se>
Date:   Thu May 14 23:33:27 2015 +0200

    curlver: introducing new version number (checking) macros