Remove last automake trace and improve acinclude.m4 header

It seems that running autoheader is unnecessary so put your hands in the
air and wave goodbye to automake.

Mention that acinclude.m4 is generated by autogen.sh.

autogen.sh

@@ -1,15 +1,15 @@
 #!/bin/sh
 
 echo acinclude.m4...
-echo "dnl This is automatically generated from m4/ files! Do not modify!" > acinclude.m4
-cat config/m4/*.m4 >> acinclude.m4
+(
+	echo "dnl Automatically generated from config/m4/ files by autogen.sh!"
+	echo "dnl Do not modify!"
+	cat config/m4/*.m4
+) > acinclude.m4
 
 echo aclocal...
 aclocal
 
-echo autoheader...
-autoheader
-
 echo autoconf...
 autoconf