Use AC_CONFIG_HEADERS.

This avoids a warning about obsolete AM_CONFIG_HEADERS() on more
recent autoconf. The new macro has been around at least since
autoconf 2.60. So this should be safe.

configure.ac

@@ -2,7 +2,7 @@ dnl Process this file with autoconf to produce a configure script. -*-m4-*-
 
 AC_INIT(src/opus_encoder.c)
 
-AM_CONFIG_HEADER([config.h])
+AC_CONFIG_HEADERS([config.h])
 
 dnl enable silent rules on automake 1.11 and later
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])