'let :=' for immediate evaluation in assignment.  Does this work for
included files?  Also ?= to only override if not already defined.

Default parameters, ie:
  goal foo (name, release = true) = ...
You might only allow defaults to be added to the end, or you
might allow goals to be called with labelled parameters.

Fix: You must use 'all ()' on the command line.

Deleting target files if goals is interrupted, but only if the
timestamp changes (what about non-*files?).  Also: atomic code.  This
will delete the target if the code doesn't run to completion.  (make
doesn't do this, but probably it should).

Parallel builds: On failure, wait for other jobs to finish.

Conditional sections (same as "ifeq" etc in make).

Let within functions and goals, eg:
  function foo () =
    let temp = "%bar%baz";
    { .... }
Unclear if this would be helpful or not.

Implement more make functions, see:
https://www.gnu.org/software/make/manual/html_node/Functions.html#Functions

Make re-execs itself if the Makefile (or any include) changes, and
goals should do something similar.  See:
https://www.gnu.org/software/make/manual/html_node/Remaking-Makefiles.html

Code should be an expression, eg this ought to work:
let foo = { echo "hello" }
and/or anonymous functions:
let foo = function (arg) { ... }

Infinite loop when you have this goal:
goal pod2man (page, section) = "docs/%page.%section" : "docs/%page.pod" { ... }
This is caused by %section matching "pod" so the rule is called
again, even if the local file docs/%page.pod actually exists.