From Gentoo bug #143804:

This may make sense for the kernel itself, but clearly does not belong in the
headers programs should use, and no installed header file actually uses
__builtin_warning. Would you please consider removing it? (I noticed it when a
non-GCC compiler complained about the syntax used.)

diff -ur linux-2.6.17/include/linux/compiler.h linux-2.6.17-gentoo/include/linux/compiler.h
--- linux-2.6.17/include/linux/compiler.h	2006-08-07 16:41:34.000000000 +0100
+++ linux-2.6.17-gentoo/include/linux/compiler.h	2006-08-13 19:11:03.000000000 +0100
@@ -26,7 +26,6 @@
 # define __iomem
 # define __chk_user_ptr(x) (void)0
 # define __chk_io_ptr(x) (void)0
-# define __builtin_warning(x, y...) (1)
 # define __acquires(x)
 # define __releases(x)
 # define __acquire(x) (void)0