PGXS does not properly uninstall documentation

Hi,

When using PGXS, putting a pathname, as opposed to
just a file name, into DOCS means that file won't
uninstall.

When installing documentation via the DOCS variable,
the Makefile runs `install`, which installs a single
file into a directory somewhere, regardless of the
source path. But the source path is what's in DOCS,
so when uninstalling the Makefile must strip the
directory part from what's in DOCS when deleting the file.

Makefile patch attached.

Regards,

Karl <kop(at)karlpinc(dot)com>
Free Software: "You don't pay back, you pay forward."
-- Robert A. Heinlein