FPC/Lazarus error - Posix.Stdlib

I'm trying to cross compile like I normally do from a Windows dev machine for Linux but I get this error

sgcHTTP_OAuth_Client.pas(173,15) Fatal: Cannot find Posix.Stdlib used by sgcHTTP_OAuth_Client of package sgcWebSocketsLazarus.

That's a Delphi specific unit.

If I add an IFNDEF like this I get another error

{$IFDEF LINUX}{$IFNDEF FPC}Posix.Stdlib, {$ENDIF}{$ENDIF}

sgcHTTP_OAuth_Client.pas(245,3) Error: Identifier not found "_system"

And the line is

_system(PIdAnsiChar('sensible-browser "' + {$IFDEF NEXTGEN}RawByteString{$ELSE}AnsiString{$ENDIF}(aURL) + '"'));