configure.in: fix autoreconf with automake-1.13
configure.in: fix autoreconf with automake-1.13

Replace long obsolete AM_CONFIG_HEADER with proper AC_CONFIG_HEADERS.
automake-1.13 errors out upon seeing former.
Similarly remove AM_PROG_CC_STDC. It's proper replacement, AC_PROG_CC,
is already present.

Signed-off-by: Marko Lindqvist <09de095ce3e68c59408f3e8a648405bbea619591@gmail.com>

johne53/MB3Gtk-2,Distrotech/gtk,johne53/MB3Gtk-2,Distrotech/gtk,Distrotech/gtk,johne53/MB3Gtk-2,Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Distrotech/gtk