Smackage should learn how to build things

[gian]

If you do (as of 1.2.1):

smackage get smbt
smackage make smbt
smackage make smbt install

You get an SML build tool that is FFI-aware, among other things. We should consider how to get smackage to make use of this (smbt needs documenting, uh... yeah...). Something like this might work:

smackage build foobar mlton

actually calls:

cd $SMACKAGE_HOME/lib/foobar/v1
smbt build.sm mlton

Which (assuming 'foobar' has an appropriate build.sm file) will do approximately the right thing. Smbt is currently smackage-aware, in the sense that it expects Smackage to tell it how to find dependencies (with the new `pathinfo' command as of 1.2.0).

Smackage need not be used with smbt in every case, but we should consider whether it could advance the cause by allowing it to be used easily, particularly given that it hides the nastiness associated with embedded FFI dependencies in libraries.