Assert that somefile.x is present after building.

[ccbrown]

If there's a problem with a build, this will save time by erroring out immediately, and making it really clear where the error occurred. Usage might look something like...

libraries:
    hiredis:
        repository: git@github.com:redis/hiredis.git
        commit: v0.13.2
        project:
            ensures: lib/libhiredis.a

[ccbrown]

It also makes sense to allow negation:

libraries:
    hiredis:
        repository: git@github.com:redis/hiredis.git
        commit: v0.13.2
        project:
            ensures:
                - lib/libhiredis.a
                - !lib/libhiredis.dylib

And in those cases, maybe Needy should just delete files that shouldn't be there.