Update configure.ac for modern autotools

AM_CONFIG_HEADERS is AC_CONFIG_HEADERS now, AM_PROG_CC_STDC is gone,
and a couple of other things are to be done differently nowadays too.

This fixes #12, thanks for @rmontagne and @3d0c for the bug report and
the help.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>