PL/PGSQL stat failed on file '$libdir/plpgsql'

Hi, 

Since Linux Mandrake 9.1 came out, I have this problem with pl/pgsql: 

[root@localhost jo]# su postgres
bash-2.05b$ createdb test
CREATE DATABASE
bash-2.05b$ createlang plpgsql test
ERROR:  stat failed on file '$libdir/plpgsql': No such file or directory
createlang: language installation failed
bash-2.05b$
bash-2.05b$ 

Does anybody know what is going on? 

Following a hint found with google: 

bash-2.05b$ pg_config --pkglibdir
bash: pg_config: command not found
bash-2.05b$ exit
exit
[root@localhost jo]# pg_config --pkglibdir
bash: pg_config: command not found 

[root@localhost jo]# echo $libdir 

[root@localhost jo]# 

What should this variable contain? 

[root@localhost jo]# cd /usr/local
[root@localhost local]# ls
bin/  doc/  etc/  games/  include/  lib/  libexec/  pgsql/  sbin/  share/  
src/  vortaro/
[root@localhost local]# cd pgsql
[root@localhost pgsql]# ls
bin/  doc/  include/  lib/  man/  share/
[root@localhost pgsql]# cd lib
[root@localhost lib]# ls
ascii_and_mic.so*     latin2_and_win1250.so*  libpq.a             
utf8_and_big5.so*      utf8_and_gb18030.so*    utf8_and_tcvn.so*
cyrillic_and_mic.so*  latin_and_mic.so*       libpq.so@           
utf8_and_cyrillic.so*  utf8_and_gbk.so*        utf8_and_uhc.so*
euc_cn_and_mic.so*    libecpg.a               libpq.so.3@         
utf8_and_euc_cn.so*    utf8_and_iso8859_1.so*  utf8_and_win1250.so*
euc_jp_and_sjis.so*   libecpg.so@             libpq.so.3.0*       
utf8_and_euc_jp.so*    utf8_and_iso8859.so*    utf8_and_win1256.so*
euc_kr_and_mic.so*    libecpg.so.3@           plpgsql.so*         
utf8_and_euc_kr.so*    utf8_and_johab.so*      utf8_and_win874.so*
euc_tw_and_big5.so*   libecpg.so.3.4.1*       utf8_and_ascii.so*  
utf8_and_euc_tw.so*    utf8_and_sjis.so*
[root@localhost lib]# 

Is everything present? 

[root@localhost lib]# su postgres
bash-2.05b$ echo $libdir 

bash-2.05b$ export libdir=/usr/local/pgsql/lib
bash-2.05b$ echo $libdir
/usr/local/pgsql/lib
bash-2.05b$ createlang plpgsql test
ERROR:  stat failed on file '$libdir/plpgsql': No such file or directory
createlang: language installation failed
bash-2.05b$ 

I'm stuck, hope anybody can help me out 

Many thanks, 

Jo