fmgr.h not found

I tried to compile something from the ../contrib-Directory and got
error messages about missing fmgr.h (lines "broken"):

----------------------------------------------------------------------
  gcc -I../../src/include -I../../src/backend   -O2  -Wall
        -Wmissing-prototypes -I ./ -I ../../src/ -I ../../src/include -I
        ../../src/port/linux -fpic -shared -o datetime_functions.so
  datetime_functions.c
  In file included from ../../src/include/access/strat.h:17,
                   from ../../src/include/utils/rel.h:18,
                   from ../../src/include/utils/builtins.h:32,
                   from datetime_functions.c:21:
  ../../src/include/access/skey.h:20: fmgr.h: Datei oder Verzeichnis
        nicht gefunden
  make: *** [datetime_functions.so] Error 1
----------------------------------------------------------------------

I located the header file in /usr/local/pgsql/include/, and after
copying it to /usr/local/src/postgresql-v6.4/src/include/, everything
works well. As I'm quite sure to not having changed anything during
the installation process (compiled it out of the tar file using the
above mentioned directories under DLD Linux 5.4, with a `make clean'
in the end), there might be a kind of bug in the installation
procedure?  I didn't find a remark about making special include
settings in the INSTALL-file (or I overlooked it ;-)

Best regards,
Ulf

--
======================================================================
 %%%%%            Ulf Mehlig              <umehlig@zmt.uni-bremen.de>
   %%%%!%%%       Projekt "MADAM"         <umehlig@uni-bremen.de>
%%%% %!% %%%%     ----------------------------------------------------
 ---| %%%         MADAM:  MAngrove    |  Center for Tropical Marine
    ||--%!%              Dynamics     |  Biology
    ||                  And           |  Fahrenheitstrasse 1
 _ /||\_/\_            Management     |
/  /    \  \ ~~~~~~~~~~~~~~~~~        |  28359 Bremen/Germany
  ~~~~~~~~~~~~~~~~~~~~