introduce option()s for backends to easily disable them

[Hugobros3]

I've been doing this by hand (editing CMakeLists.txt) for years, but I think we need a proper way to do this: disable backends at will, easily. For various reasons (LLVM linking conflicts, old broken installations for some dependency, buggy backend, ...) this is often required and thus needs to be facilitated.

This arguably also addresses the motivation for https://github.com/AnyDSL/runtime/pull/51 by only attempting to find_package if a given backend is enabled.

Currently this defaults to try to enable all backends, before looking for their dependencies.

Alternatively we could:

• Fail if a dependency is not meant but the backend is enabled
• Do find_package first and remove the option if the dependencies aren't met
• Forcefully disable backends when depdencies aren't met (with set(... FORCE))