* m4sugar.m4 (m4_undefine, m4_popdef): Don't tolerate undefined

* m4sugar.m4 (m4_undefine, m4_popdef): Don't tolerate undefined
arguments.
(_m4_expansion_stack): Rename as...
(m4_expansion_stack): this, and change its value: instead of using
the pushdef stack to stack each *line* of the stack, each
definition contains the whole stack.  I.e., to display the whole
stack, instead of popdefing and displaying each definition, just
display the current definition.
(m4_expansion_stack_push, m4_expansion_stack_pop): New.
* tests/atspecific.m4 (AT_CHECK_AUTOCONF): Let $2 be the expected
exit status.
* tests/m4sugar.m4 (m4_require: circular dependencies): New test.