Improve `make -n dist' and `make -n distcheck' for GNU make. Ensure that `make -n dist' and `make -n distcheck' do not change files, due to GNU make executing rules containing `$(MAKE)'. * lib/am/distdir.am (distdir): Separate the creation of `$(distdir)/$$subdir' for `$(DIST_SUBDIRS)' and the recursion into the `$(DIST_SUBDIRS)' in two separate rule commands. (distcheck): Exit recursive rule early when run with `make -n', as detected by a witness file. * tests/maken.test, tests/maken2.test: New tests. * tests/Makefile.am: Update. * NEWS: Update. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>