Always Enable Installation Target

[threeal]

This issue suggests always enabling the installation targets in this sample project. This will essentially revert the changes proposed in #23, which disabled the installation targets when this sample project was treated as a subproject.

The installation target is proposed to always be enabled because there is no reason to disable it, even for a subproject. Targets inside a subproject also need to be installed, contrary to the view described in #23, which stated that installation targets inside a subproject do not need to be installed.