Missing b2 option to disable boost_cobalt_io_ssl & SSL in general

Hello,

I was lately running into problems building boost (1.90.0). Using conan as a package manager, I have been able to easily use boost so far. But with 1.90.0 and the addition of boost_cobalt_io_ssl, some problems started to appear. They are related to: Boost looking for a SSL library and building cobalt_io_ssl.

If no ssl is found (before I added libssl-dev to my Dockerfile), it build well, didn't found ssl & skipped cobalt_io_ssl. As soon as the system lib is there, it gets linked which is unwanted. For other libraries like zlib, lzma etc. there is flags to just deactivate them. Like `-sNO_ZLIB=1` or `-sNO_ZSTD=1` for example. So I'd say there should be a similar option to force SSL off.

Also, there seems to be no option to just disable cobalt_io_ssl from being built? Am I missing something or is that really missing?

The full details in the conan repo issue: https://github.com/conan-io/conan-center-index/issues/29433