Re: [PATCH] use m4_require to implement AS_REQUIRE

[Paolo Bonzini]

> I still don't quite understand (but at this point have to admit that I
> haven't looked at the implementation _at all_).  If AC_CHECK_FUNC
> requires the preparation of AC_LINK_IFELSE, then the latter should be
> expanded outside and before the body of the former.  No?

After this patch, yes. :-)

Without it, AS_REQUIRE is a hack that does not guarantee "atomic"
expansion of macros; it just slapped text into the appropriate
diversion, causing intermixed expansion like this one:

   ac_c_check_func() {
     if test $ac_cv...; then
       ...
     else
       ac_c_link_ifelse() {
       }
     fi
  }

and requiring commits like 34b79a8 and 926ed6a9 which could even be
reverted after this patch goes in (though I'd rather not do it to avoid
excessive churn -- there is more than enough already).  Look at those
seemingly innocent patches and you'll realize that, while tolerable, the
situation was anyway all but optimal.

> Ralf (and yes, just ignore me unless you have extra time to explain)

No problem.  Explaining things twice makes sure you have really
understood them.  Also, since I submitted the first shell function
patches 4-5 years ago, and so many prerequisites jump up now years
later, I'd better explain things as much as possible to make sure I'm
doing them right.

Paolo