Perhaps adjusting the compiler defines is enough. Don't have a wince device handy, typing this in the post and haven't even compiled, so no guarantees:
unit synaser;
interface
uses
{$IFDEF MSWINDOWS}
// Any Windows version (Delphi+FPC define)
// Windows desktop x86, x64, Windows CE
Windows, registry,
{$ELSE}
// Linux, Unix
{$IFNDEF NO_LIBC}
Libc,
KernelIoctl,
{$ELSE}
termio, baseunix, unix,
{$ENDIF}
{$IFNDEF FPC}
Types,
{$ENDIF}
{$ENDIF} //MSWINDOWS
{$IFDEF FPC}
winver,
{$ENDIF}
{$ENDIF}
synafpc,
Classes, SysUtils, synautil;