configure: Add AM_MAINTAINER_MODE option

fixes the message

configure: WARNING: unrecognized options: --enable-maintainer-mode

at the end of configure run when one was using autogen.sh.

Patch by Stefan Kost

git-svn-id: https://svn.xiph.org/trunk/Tremor@17157 0101bb08-14d6-0310-b084-bc0e0c8e3800

configure.in

@@ -13,6 +13,9 @@ AM_CONFIG_HEADER([config.h])
 
 AM_INIT_AUTOMAKE(libvorbisidec,1.2.0)
 
+dnl AM_MAINTAINER_MODE only provides the option to configure to enable it
+AM_MAINTAINER_MODE
+
 dnl Library versioning
 
 V_LIB_CURRENT=1