[fpc-pascal] Using platform specific include files like FPC does

[Marco van de Voort]

In our previous episode, Graeme Geldenhuys said:
> Ah, so that's the secret. :-)  Thanks, I'll try this one. I'm pretty
> sure I can use some of the built-in path macros in Lazarus IDE to
> accomplish this.

Afaik lazarus already does this in de lcl/ dir.
 
> > Note that afaik the main reason FPC does this is because too many ifdefs
> > make the source unreadable. It is not because of a grudge against ifdefs.
> 
> An the same reason I want to get rid of IFDEFs. The obfuscate the code.

(I meant more that we only do it for the heaviest ifdefed files)