fail and warnings for build configurations

[duncan]

People expect buildable: False to fail the current configuration (rather than letting it configure and then not working at build time). But perhaps we should make that more explicit and allow a message, eg instead of:

if os(windows)
  buildable: False

allow

if os(windows)
  fail: it doesn't work on windows because, blah blah blah

And similarly we might want to warn users about selecting particular configurations eg:

if flag(http)
  ...
else
  warn: no http implementation selected, only local
        operations will be possible. You probably really
        want some http implementation.

The semantics roughly is that it's a workable but sub-optimal configuration.

The solver should try to avoid warnings, by selecting another alternative.