Compilation of C function on Windows

Hi, does anyone have experience in compiling c function into *.so file?

That is, it's making that "unixtime.so" file that I'm stuck on at
the moment in below codelet.
CREATE FUNCTION unixtime()
RETURNS int4
AS '/opt/postgres/lib/unixtime.so'
LANGUAGE 'c' ;

Any comment would help me a lot. THanks!