When encountering differences in Delphi/FPC calls, the best thing to do is not to insert IFDEF statements whenever a difference is encountered, but to create a separate unit which is only used when compiling with FPC. The missing/incompatible calls can then be implemented in that unit. This will keep the code more readable and easier to maintain.
If a language construct difference is found, then the Free Pascal team should be contacted and a bug should be reported.