Run stock installation logic after before_install (or other chunk of user code)

As it is now, stock installation logic (platform install, build addons) run before any user-supplied code gets control. As a consequence, if that logic doesn’t quite fit the needs or something goes wrong with it, all a user can do is ditch it altogether – they can’t do anything about the problem.

I’m suggesting to run some user-supplied code before any stock logic fires, except perhaps cache restoration. before_install looks like the perfect candidate: before_install: – stock logic – install: will give the user control over both the stock logic’s inputs and outputs – thus the theoretical ability to alter it any way they need.

See https://github.com/matthew-brett/multibuild/issues/116 for the same problem in another project that was fixed this way.