pgsql: Fix some oversights in distprep and maintainer-clean targets.

Fix some oversights in distprep and maintainer-clean targets.

At least two recent commits have apparently imagined that a comment in
a Makefile stating that something would be included in the distribution
tarball was sufficient to make it so. They hadn't bothered to hook
into the upper maintainer-clean targets either. Per bug #5923 from
Charles Johnson, in which it emerged that the 9.1alpha4 tarballs are
short a few files that should be there.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/174f65ab00bb8de0f119a6a60d562b516ba71bba

Modified Files
--------------
src/Makefile | 5 +++++
src/backend/Makefile | 6 +++++-
src/test/isolation/Makefile | 6 ++++--
3 files changed, 14 insertions(+), 3 deletions(-)