BUG #5595: Documentation is not installs from VPATH build.

The following bug has been logged online:

Bug reference:      5595
Logged by:          Dmtiriy Igrishin
Email address:      dmitigr@gmail.com
PostgreSQL version: 9.0 beta4
Operating system:   Ubuntu Linux 9.04 (2.6.31-22 x86_64)
Description:        Documentation is not installs from VPATH build.
Details:

When 'configure' executed in a directory outside the source tree the
documentation is not installs later nevertheless the "gmake install-docs" or
"gmake install-world" (I tried to build with "gmake world" also) typed to
install PostgreSQL.
When 'configure' executed inside the source tree - all is okay.