tool_help: Warn if curl and libcurl versions do not match

.. because functionality may be affected if the versions differ.

This commit implements TODO 18.7 "warning if curl version is not in sync
with libcurl version".

Ref: https://github.com/curl/curl/blob/curl-7_64_1/docs/TODO#L1028-L1033

Closes https://github.com/curl/curl/pull/3774