Enable maintainer mode by default on automake 1.11

Passing 'enable' as an argument to AM_MAINTAINER_MODE flips the
default to enabled, rather than disabled until automake 1.11.
This is a safer default for a developer-oriented library.

./configure --disable-maintainer-mode is still available for
packagers who what to preserve the upstream build distribution.

On earlier automake versions, the argument appears to have no
effect, so there is no behaviour change for developers using
older autotools.

Based on a patch for opusfile by ron@debian.org.

configure.ac

@@ -62,7 +62,7 @@ AC_SUBST(OPUS_LT_REVISION)
 AC_SUBST(OPUS_LT_AGE)
 
 AM_INIT_AUTOMAKE($PACKAGE, $VERSION, no-define)
-AM_MAINTAINER_MODE
+AM_MAINTAINER_MODE([enable])
 
 AC_CANONICAL_HOST
 AC_MINGW32