add more instruction on building s4

[Matthias Dieter Wallnöfer]

Hi Matthieu!

+ "./autogen.sh": This script is already described a few lines earlier 
(in the sense as "if you don't use a packaged version you first need to...")

+ "make idl_full": You don't really need to use this - it works also 
without in most cases ("make" should recompile the IDLs when needed). 
Only under special circumstances (major IDL changes) this could be 
useful - but then it isn't said that the normal build works - so you 
probably anyway need the full rebuild.

So I think this patch isn't really needed.

Matthias

Matthieu Patou wrote:
> See the patch.
>
> Matthieu.