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)

 diff -durN glibc-2.10.1.orig/sysdeps/sh/sh4/fpu/fpu_control.h glibc-2.10.1/sysdeps/sh/sh4/fpu/fpu_control.h

 --- glibc-2.10.1.orig/sysdeps/sh/sh4/fpu/fpu_control.h	2001-07-06 06:56:04.000000000 +0200

 +++ glibc-2.10.1/sysdeps/sh/sh4/fpu/fpu_control.h	2009-11-13 00:51:26.000000000 +0100

 @@ -45,6 +45,7 @@

  #define _FPU_GETCW(cw) __asm__ ("sts fpscr,%0" : "=r" (cw))

  #if defined __GNUC__

 +extern void __set_fpscr(unsigned long);

  #define _FPU_SETCW(cw) __set_fpscr ((cw))

  #else

  #define _FPU_SETCW(cw) __asm__ ("lds %0,fpscr" : : "r" (cw))