Enable maintainer mode for sim

Enable maintainer mode for sim

The sim's recently switch to using Automake caused a build failure for
me, because I didn't have the correct auto* tools in my path.
However, the rule in the tree is that this is not needed in general.
This patch adds a call to AM_MAINTAINER_MODE, to align the sim with
the way the rest of the tree works here.

sim/ChangeLog
2021-03-08  Tom Tromey  <tromey@adacore.com>

	* aclocal.m4, configure, Makefile.in: Rebuild.
	* configure.ac: Use AM_MAINTAINER_MODE.