Implement AM_MAINTAINER_MODE([DEFAULT-MODE]). * m4/maintainer.m4 (AM_MAINTAINER_MODE): Allow for an optional first argument of `enable' or `disable', setting the default. * doc/automake.texi (Optional, maintainer-mode): Document. * tests/mmode.test: New test. * tests/Makefile.am: Adjust. * NEWS: Update. Suggestion by Vincent Lefevre. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>