More macro dispatching

[Akim Demaille]

In order to properly implement the proposal of Alexandre (expr=false
when we can't trust it), more responsibility should be moved into
Mash, in particular things like _AC_INIT_PREPARE_ENVIRONMENT.  It is a
good thing, it is independent from Autoconf.

But to be really clean, some form of AC_REQUIRE would be good too.

So I'm proposing to move the AC_REQUIRE, AC_DEFUN and company into
m4sugar.  Again, I think this is a good thing, some other projects
than Autoconf might want it.  It'd be m4_defun, m4_require etc.

How does it sound?