Avoid installation if included as a subproject

[nicola-gigante]

When clipp is included as a subproject with add_subdirectory, the target is installed alongside the other targets of the main project. This is not always advisable, especially not if the main project installs an executable (so the headers file are not needed for users of the main target).

This PR provides a variable that the user can set to enable the install target. The variable is set by default on ON or OFF depending on whether the project is being built as a subproject or not.

A potential problem might arise for compatibility with existing clients that implicitly relied on the current behavior. They would have to explicitly set the CLIPP_INSTALL variable to ON. If this breaking change has to be avoided, another acceptable solution would be to simply set the variable to ON by default and at least let the client disable it if needed.