* m4/amversion.in (_AM_AUTOCONF_VERSION): New macro.

* m4/amversion.in (_AM_AUTOCONF_VERSION): New macro.
(AM_SET_CURRENT_AUTOMAKE_VERSION): Call it.
* aclocal.in (trace_used_macros): Trace _AM_AUTOCONF_VERSION.
(write_aclocal): Output a check for Autoconf's version in aclocal.m4.
Doing so ensures that users cannot build configure and Makefiles
with two different autoconf versions.  Report from Noah Misch.
* tests/missing4.test: New file.
* tests/Makefile.am (TESTS): Add it.