Stop using old AM_CONFIG_HEADER macro

Building with automake-1.13 throws an error:
configure.ac:6: error: 'AM_CONFIG_HEADER': this macro is obsolete.
    You should use the 'AC_CONFIG_HEADERS' macro instead.

* configure.ac (AM_CONFIG_HEADER): Rename to AC_CONFIG_HEADERS.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/configure.ac b/configure.ac
index e4cd788..28bf86c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -5,7 +5,7 @@
 	[strace-devel@lists.sourceforge.net])
 AC_CONFIG_SRCDIR([strace.c])
 AC_CONFIG_AUX_DIR([.])
-AM_CONFIG_HEADER([config.h])
+AC_CONFIG_HEADERS([config.h])
 AM_INIT_AUTOMAKE([foreign dist-xz no-dist-gzip silent-rules])
 AM_MAINTAINER_MODE
 AC_CANONICAL_HOST