build: Remove deprecated AM_CONFIG_HEADER

AM_CONFIG_HEADER has been deprecated for many years and is removed
completely from automake 1.13.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/configure.ac b/configure.ac
index ff34f03..51bc8bd 100755
--- a/configure.ac
+++ b/configure.ac
@@ -19,7 +19,7 @@
 AC_CANONICAL_TARGET()
 
 AM_INIT_AUTOMAKE(1.6)
-AM_CONFIG_HEADER(config.h)
+AC_CONFIG_HEADERS(config.h)
 
 AC_PATH_PROG(PERL, perl)
 AC_CHECK_PROG([GAWK],[gawk],[gawk],[not-in-PATH])