New custom dependency for OpenSSL

Detecting an OpenSSL installation in a cross-platform manner can be complicated. Officially, pkg-config is supported by upstream. Unofficially, cmake includes a FindOpenSSL using a different name and which requires specifying modules.

Meson will now allow the pkg-config name to work in all cases using the following lookup order:

• prefer pkg-config if at all possible
• attempt to probe the system for the standard library naming, and retrieve the version from the headers
• if all else fails, check if cmake can find it