Add AM_MAINTAINER_MODE

and keep it enabled by default. This allows downstreams to turn it off
via:

./configure --disable-maintainer-mode

as discussed in

https://www.redhat.com/archives/virt-tools-list/2010-October/msg00049.html

configure.ac

@@ -7,6 +7,7 @@ AC_CONFIG_HEADERS([config.h])
 AC_CONFIG_MACRO_DIR([m4])
 dnl Make automake keep quiet about wildcards & other GNUmake-isms
 AM_INIT_AUTOMAKE([-Wno-portability])
+AM_MAINTAINER_MODE([enable])
 
 # Use the silent-rules feature when possible.
 m4_ifndef([AM_SILENT_RULES], [m4_define([AM_SILENT_RULES],[])])