Bass_init simply requires a window handle (HWND).
The person who made/converted the headers simply assumed that when you are not on windows, there is no HWND, rather a pointer, Quite possibly that person was not aware of the existence of other platforms that also uses HWND handles.
{$IFDEF MSWINDOWS}
function BASS_Init(device: Integer; freq, flags: DWORD; win: HWND; clsid: PGUID): BOOL; stdcall; external bassdll;
{$ELSE}
function BASS_Init(device: Integer; freq, flags: DWORD; win: Pointer; clsid: Pointer): BOOL; cdecl; external bassdll;
{$ENDIF}
If this is considered difficult, then be prepared because the sources of those headers are riddled with this kind of wrongful assumptions which makes your BASS life a little less soundy