BUG #1766: contrib/ modules can't install with --without-docdir

The following bug has been logged online:

Bug reference:      1766
Logged by:          ISHIDA Akio
Email address:      iakio@mono-space.net
PostgreSQL version: 8.0.3
Operating system:   Linux
Description:        contrib/ modules can't install with --without-docdir
Details:

$ ./configure --without-docdir
..
$ cd contrib/pgstattuple/
$ make install
mkdir -p -- /contrib
mkdir: cannot create directory `/contrib': Permission denied
make: *** [installdirs] Error 1


--- src/makefiles/pgxs.mk.org   2004-10-11 01:13:03.000000000 +0900
+++ src/makefiles/pgxs.mk       2005-07-14 09:54:24.000000000 +0900
@@ -100,10 +100,12 @@
        done
 endif # MODULES
 ifdef DOCS
+ifdef docdir
        @for file in $(addprefix $(srcdir)/, $(DOCS)); do \
          echo "$(INSTALL_DATA) $$file $(DESTDIR)$(docdir)/contrib"; \
          $(INSTALL_DATA) $$file $(DESTDIR)$(docdir)/contrib; \
        done
+endif # docdir
 endif # DOCS
 ifdef PROGRAM
        $(INSTALL_PROGRAM) $(PROGRAM)$(X) $(DESTDIR)$(bindir)
@@ -133,8 +135,10 @@
        $(mkinstalldirs) $(DESTDIR)$(pkglibdir)
 endif
 ifdef DOCS
+ifdef docdir
        $(mkinstalldirs) $(DESTDIR)$(docdir)/contrib
-endif
+endif # docdir
+endif # DOCS
 ifneq (,$(PROGRAM)$(SCRIPTS)$(SCRIPTS_built))
        $(mkinstalldirs) $(DESTDIR)$(bindir)
 endif