distdir: avoid duplicate `find' traversal with subpackages. * lib/am/distdir.am (distdir) [%?SUBDIRS%]: When recursing, set `am__skip_mode_fix'. [%?TOPDIR_P%]: When `$(am__skip_mode_fix)' is set, avoid tree walk. * tests/subpkg4.test: New test, to ensure that we still walk the whole tree while fixing permissions. We don't ensure it is walked once only. * tests/Makefile.am: Update. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>