scripts: unset CONFIG_SITE

Some distributions (eg. openSUSE 12.1) systematically export
the CONFIG_SITE environment variable to point to a custom
script setting misc paths for ./configure.

This can, and does, break when cross-compiling for architectures
that are not supported by this script.

The simple workaround is to unset this variable.
NB: buildroot has a similar fix:
http://git.buildroot.org/buildroot/commit/?id=12c9f7dd6dee9c6029b4f9a12d6aac1516911ab4

Reported-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 21f4f28e60ec0342133086c9a51e7f0e5b181fb8)

 --- glibc-2.7/sysdeps/unix/sysv/linux/kernel-features.h	2007-08-14 06:21:09.000000000 +0300

 +++ glibc-2.7/sysdeps/unix/sysv/linux/kernel-features.h.new	2008-07-23 07:05:20.000000000 +0300

 @@ -449,8 +449,7 @@

     and on Alpha just after 2.6.22-rc1.  */

  #if __LINUX_KERNEL_VERSION >= 0x020611 \

      && ((!defined __sh__ && !defined __alpha__) \

 -	|| (__LINUX_KERNEL_VERSION >= 0x020613 && defined __sh__) \

 -	|| (__LINUX_KERNEL_VERSION >= 0x020617 && defined __alpha__))

 +	|| (__LINUX_KERNEL_VERSION >= 0x020613 && defined __sh__))

  # define __ASSUME_ATFCTS	1

  #endif