kmk: Add generic loop functions $(for ) and $(while ).

[bird]

$(for init,condition,next,body) The init part is evaluate up front (no extra expansion). The condition part is a kmk-expression and is evaluate before each iteration. The body is expanded and added to the output, the output from each iteration is separated by a space. The next part is evaluated at the end of each iteration (no extra expansion).

$(while condition,body - Similar to $(for ) with empty init and next.

[bird]

(In [2161]) kmk: Implemented $(for ) and $(while ) loops (C-style). Fixes #73.