[configure] Make maintainer mode default to on but not explicit in autogen

Maintainer mode is assumed on when executing configure, and thus is not needed
within autogen.

autogen.sh

@@ -2,4 +2,4 @@
 
 (cd $(dirname $0);
  autoreconf --install --symlink &&
- ./configure --enable-maintainer-mode $@)
+ ./configure $@)

configure.ac

@@ -12,7 +12,7 @@ AC_C_CONST
 
 AM_INIT_AUTOMAKE([dist-bzip2 no-dist-gzip])
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
-AM_MAINTAINER_MODE
+AM_MAINTAINER_MODE([enable])
 
 PKG_PROG_PKG_CONFIG
 AM_PROG_LIBTOOL