The current implementation of AC_REQUIRE fails on

The current implementation of AC_REQUIRE fails on

| AC_DEFUN([TEST1],  [REQUIRE([TEST2a])REQUIRE([TEST2b])])
| AC_DEFUN([TEST2a], [])
| AC_DEFUN([TEST2b], [REQUIRE([TEST3])])
| AC_DEFUN([TEST3],  [REQUIRE([TEST2a])])
|
| AC_INIT
| TEST1

because it produces TEST3; TEST2a; TEST2b; TEST1.
Fix this bug, implement the solution provided by Axel Thimm,
and test AC_REQUIRE.

* acgeneral.m4: Document this implementation.
(_AC_DEFUN_PRO, _AC_DEFUN_EPI, AC_REQUIRE): Be sure that macros
are emitted in the same order as they are expanded.
(AC_REQUIRE): Forbid being calling out of an AC_DEFUN'd	macro (in
particular the top level).
* tests/base.m4 (AC_REQUIRE): New test.