Ok, thanks a lot for the tips, RPM seems more friendly to me now
ps : I've found the exact command :
$ rpm --recompile util-linux-2.12a-5mdk.src.rpm
which leaves the sources and a prepared Makefile in the following directory : /usr/src/RPM/BUILD/util-linux-2.12a/
Why the hell am I always doing it the hard way...???
Oh yes... I don't read the docs.