cmake: offer command-line abstraction for install scripts

Currently one may use cmake --build . --target install to build and install.

!3069 (merged) proposes a cmake --install . variant. Discussion there raised a few points that would be better worked out in an issue first, so I'm opening this.

Concerns include:

• cmake --install . is a sensible abstraction over the current cmake ... -P cmake_install.cmake support.
• When does installation also build?
• Re-linking (make preinstall) may be needed before directly invoking cmake_install.cmake scripts.
• The Ninja generator does not provide the preinstall target. This is an existing limitation that should be worked out regardless of the rest of this issue.