configure.in: Remove obsolete macro usage

Use AC_CONFIG_HEADERS instead of AM_CONFIG_HEADER, and drop the use of
AM_PROG_CC_STDC, because these macros are long obsolete, and were
removed from automake 1.13.

Reported-by: Michael Sterrett <mr_bones_@gentoo.org>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>