Certain subdirectories do not get built if corresponding options are not
selected at configure time.  However, "make distprep" should visit such
directories anyway, so that constructing derived files to be included in
the tarball happens without requiring all configure options to be given
in the tarball build script.  Likewise, it's better if cleanup actions
unconditionally visit all directories (for example, this ensures proper
cleanup if someone has done a manual make in such a subdirectory).

To handle this, set up a convention that subdirectories that are
conditionally included in SUBDIRS should be added to ALWAYS_SUBDIRS
instead when they are excluded.

Back-patch to 9.1, so that plpython's spiexceptions.h will get provided
in 9.1 tarballs.  There don't appear to be any instances where distprep
actions got missed in previous releases, and anyway this fix requires
gmake 3.80 so we don't want to apply it before 9.1.