Install only defined if lexy is top level project

[adamdusty]

Is there a reason that all the install rules are behind an if statement checking if source_dir is current_source_dir?

This makes it so if I use lexy in a library that also defines install targets, I have to manually include every lexy target (of which there are a few).

[foonathan]

I didn't imagine that people used it as add_subdirectory and then want to install their project. I thought for that use case you want to install lexy first, then call find_package.

But I welcome a PR that moves the install rules.