Install hack workaround for failure of 'make all' in VPATH builds.

It appears that gmake gets confused if postgres.sgml is not present in
the working directory, and instantiates some default rule or other that
would let postgres.sgml be built from postgres.xml.  I haven't been able
to track down exactly where that's coming from, but the problem can be
dodged by specifying srcdir explicitly in the rule for postgres.xml.
Per report from Vladimir Kokovic.