Build: Makefile.am: add ACLOCAL_AMFLAGS = -I m4

Per the libtoolize documentation: "In the future other Autotools will
automatically check the contents of AC_CONFIG_MACRO_DIRS, but at the moment it
is more portable to add the macro directory to ACLOCAL_AMFLAGS in Makefile.am."

This was why version.m4 had to be in the top-level directory.