support for precompiled headers

[Michal H. <ms...@gm...>]

Jozo suggested using of precompiled headers in kernel code.
(here is some reference http://gcc.gnu.org/onlinedocs/gcc/Precompiled-Headers.html)

Basically, I am not against this idea, but I don't like to use it right
now. 
	- There are many limitations (see above list) 
	- not all compilers come with this feature (gcc > 3.4.0, don't 
	  know about other compilers and platforms with gcc)
	- it needs to change our makefiles to acomplish
	- kernel code is not so big and compilation time is minor when
	  comparing to gui (and gui doesn't benefit from it at all IIUC)

But we can discuss it and add it later.
-- 
Michal Hocko