under libdir, for a cleaner separation in the installation layout
and compatibility with binary packaging standards. Point backend's
default search location there. The contrib modules are also
installed in the said location, giving them the benefit of the
default search path as well. No changes in user interface
nevertheless.
Client headers are no longer in a subdirectory, since they have been made
namespace-clean.
Internal libpq headers are in a private subdirectory.
Server headers are in a private subdirectory. pg_config has a new option
to point there.
