plpython thinks it's hooked into "make distprep", but not so much

I noticed while wrapping 9.1beta2 that plpython tries to build a file
spiexceptions.h to be included in the tarballs, but no such file is
actually appearing therein. The reason is that src/pl/Makefile doesn't
recurse into the plpython subdirectory unless with_python is set.
Which it isn't, because the tarball build script doesn't configure
--with-python (or with any other options for that matter). If it did,
it would fail, since there's no python installation on
developer.postgresql.org.

I left this go for the moment, since the only implication is that you
have to have perl available to build plpython from the tarball. But we
oughta fix it before final. Any opinions on what's the least ugly way
to get control to recurse to src/pl/plpython for "make distprep"
regardless of with_python?

(More generally, I would like to see cleanup targets such as make
distclean and make maintainer-clean recurse to all subdirectories
regardless of configure options, but that might be too much to ask for
right now.)

regards, tom lane