Automatically uninstall before install

[gergondet]

This PR automatically calls uninstall before an install command is called.

• The motivation is to avoid keeping stale libraries and obsolete header files around
• This relies on the documented fact that install commands are run in the specified order, hence the "hook" is setup early, normally before any call to install in user code happened (i.e. as long as you don't have install commands before include(base.cmake) it should work
• it changes the behavior of the uninstall target so that it doesn't print an error message if called before install (for the first time)

Since it's an impactful change I will wait for feedback (@nim65s @jcarpent @wxmerkt in particular) before merging

[gergondet]

Thanks for the feedback, I'll merge this now.