cg_let macro for evaluating m4 code with definitions provided as an associative list

```
A cg_let(<alist>, <code>) macro could expand to <code>,
wrapped with a number of m4_pushdef/m4_popdef's,
taking their definitions from <alist>.
A three-arguments form could accept a prefix for the macros,
so that an arbitrary alist can be used without polluting the namespace.

```

Original issue reported on code.google.com by `jeremie....@gmail.com` on 22 Jun 2009 at 3:45
