Allow package trees (packages with subpackages) to share

Allow package trees (packages with subpackages) to share
common distributed auxiliary files (and directories) that
reside within a subpackage.

* lib/am/distdir.am (distdir %?TOPDIR_P%): Do not fail if
`$(distdir)' already exists.
(%?SUBDIRS%): Set `am__remove_distdir' to `:' to prevent
removal of subpackage trees for distribution.
* tests/subpkg3.test: New test.
* tests/Makefile.am: Update.