Squash Debian/Ubuntu build warning.

Debian (and Ubuntu) add --disable-maintainer-mode to configure
when building packages, leading to a unrecognised argument warning.

Adding AM_MAINTAINER_MODE disabled keeps the current behavior while
removing the warning.

configure.ac

@@ -15,6 +15,7 @@ OCFLAGS="$CFLAGS"
 AC_CANONICAL_SYSTEM
 AC_LANG(C)
 
+AM_MAINTAINER_MODE([disable])
 AM_INIT_AUTOMAKE([1.11 foreign color-tests parallel-tests])
 AM_SILENT_RULES([yes])
 AC_DISABLE_STATIC