pgsql: Don't auto-create the subdirectories holding built documentation

Log Message:
-----------
Don't auto-create the subdirectories holding built documentation in a VPATH
build tree. If we actually build the docs in the VPATH tree, those dirs
will get created then; but if they're present and empty, they capture the
vpathsearch searches in "make install", preventing installation of prebuilt
docs that might exist in the source tree. Per bug #5595 from Dmtiriy Igrishin.
Fix based on idea from Peter Eisentraut.

Tags:
----
REL9_0_STABLE

Modified Files:
--------------
pgsql/config:
prep_buildtree (r1.5 -> r1.5.32.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/config/prep_buildtree?r1=1.5&r2=1.5.32.1)