build: use dnl for comments in configure.ac

This simply means the comments are not copied into the executed
configure script.

configure.ac

-#                                               -*- Autoconf -*-
-# Process this file with autoconf to produce a configure script.
+dnl                                               -*- Autoconf -*-
+dnl Process this file with autoconf to produce a configure script.
 
 AC_INIT([fdk-aac], [0.1.0], [http://sourceforge.net/projects/opencore-amr/])
 AC_CONFIG_AUX_DIR(.)
@@ -8,16 +8,16 @@ AM_INIT_AUTOMAKE([tar-ustar])
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 AC_CONFIG_HEADERS([config.h])
 
-# Checks for programs.
+dnl Checks for programs.
 AC_PROG_CXX
 
-# Setup for libtool
+dnl Setup for libtool
 AC_PROG_LIBTOOL
 AC_SUBST(LIBTOOL_DEPS)
 
-# soname version to use
-# goes by ‘current[:revision[:age]]’ with the soname ending up as
-# current.age.revision.
+dnl soname version to use
+dnl goes by ‘current[:revision[:age]]’ with the soname ending up as
+dnl current.age.revision
 FDK_AAC_VERSION=0:1:0
 AC_SUBST(FDK_AAC_VERSION)