"fix" <let>-bound lambda expressions too

"fix" <let>-bound lambda expressions too

* module/language/tree-il/compile-glil.scm (compile-glil): Compute
  warnings before optimizing, as unreferenced variables will be
  optimized out.

* libguile/_scm.h: Fix C99 comment.

* module/language/tree-il/fix-letrec.scm (partition-vars): Also analyze
  let-bound vars.
  (fix-letrec!): Fix a bug whereby a set! to an unreffed var would be
  called for value, not effect. Also "fix" <let>-bound lambda
  expressions -- really speeds up pmatch.

* test-suite/tests/tree-il.test ("lexical sets", "the or hack"): Update
  to take into account the new optimizations.