Maybe you are right, they could put in some directives to link only the versions you need, i think there should be no problem to use them with FPC since they work in Kylix, however FPC has some nice macros to avoid all those IFDEFS (don't work in Delphi ), it simply declares a dummy calling convention and then it swiches it based on the OS defines, i love those macros .
I was thinking that if i have enough time i will make my own based on both headers and include my own tricks in them.