Tests done using 'aclocal -I m4' in buildconf and 'ACLOCAL_AMFLAGS = -I m4

Tests done using 'aclocal -I m4' in buildconf and 'ACLOCAL_AMFLAGS = -I m4
in top Makefile.am triggered a problem that prevented aclocal from running
successfully on SunOS 5.10 with GNU m4 1.4.5 and GNU Autoconf 2.61

A tarball which reproduces mentioned problem is the one dated July-28-2008
http://cool.haxx.se/curl-daily/curl-7.19.0-20080728.tar.gz

We actually don't need all the bells and whistles that the above mechanism
provides. We only need to include our m4/reentrant.m4 file in acinclude.m4
so here we go with this simpler mechanism.