OK,
after searching mailing list archives I've found on 
*egcs* :

----------------------------------------------------
> looking thru xf86 config, I've seen that
> they use
> -O2 -fno-strength-reduce
> 
> `to work around a bug in gcc2'.

What version of XFree86 do you have?

The bug in question was fixed in version 2.7.2.1, long before the egcs
project was even started.  Many autoconf-based programs include a test
for the bug that is run by "configure", so that the flag can be added
if someone is running an older compiler.
--------------------------------------------------------

So if you have gcc 2.7.2.>=1 or egcs you can build with -O2
(i hope so), at least i'm going to build XF86 3.3.3 once more.

cheers