cmake: add CURL_ENABLE_SSL option

[nightlark]

I came across a project building libcurl as a subproject that was trying to set SSL_ENABLED to OFF to disable SSL support in libcurl. Other than SSL_ENABLED being set as a local variable in libcurl and basically overriding anything that they might have set in the cache, issues would have also been encountered because CMAKE_USE_OPENSSL defaults to ON.

I think adding an ENABLE_SSL option (maybe renamed to match any curl cmake option naming convention) that the various SSL backends also depend on would make it much easier for projects building libcurl as a subproject to disable ssl without needing to determine which CMAKE_USE_* variables for SSL backends are set, and more obvious what option needs to be turned ON/OFF.

By ENABLE_SSL defaulting to ON, the behavior of existing SSL related options should remain the same and avoid breaking any existing ci setups or build scripts.

Thoughts?

[jzakrzewski]

Please namespace the option. ENABLE_SSL is in my opinion too generic and may cause clashes with other projects. I myself have for example included MongoDB driver and curl in the same project and that alone would cause problems with this change.

[nightlark]

Option name is now under the CURL_ namespace.

[jzakrzewski]

No time to test but at least looks sensible ;)

[MarcelRaad]

The test failures look unrelated.

[MarcelRaad]

Merged now. Thank you!