'Undefined reference' error message when linking an object file

["VOGLOZIN W. Amenel" <amenel(dot)voglozin(at)univ-nantes(dot)fr>]

Hi,
I'm currently developing an access method for a research project, and I  
need some experimental measures. So I wrote an extension for PostgreSQL,  
with all functions in a single C file.
I built Postgresql 8.1.3 from sources and installed it using MSYS.

Following the instructions at section 32.9 of the manual  
(http://www.postgresql.org/docs/8.1/static/xfunc-c.html#DFUNC), I compiled  
the C file without any message. But I just can't succeed in producing the  
shared object using MinGW & MSYS. No matter what library directory I  
indicate, I still get the same set of "undefined reference to ..."  
messages. I know this means a library is not found.
The command line I'm using is:

gcc -L/local/pgsql/lib -shared seqsumproc.o

So my questions are :
1- is there something wrong in the command line I'm using above ?
2- what is the name of the library that contains all server functions ?
3- I read from other mails on this mailing-list that the shared library  
produced by MinGW is a DLL. Is the extension automatically given or should  
I rename the file later ?

I read as much as I could to get a solution before asking here, including  
this mailing-list's archive.
Thanks for reading and any help.