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_9.orig/nptl/init.c	2007-10-15 22:25:25.000000000 +0200

+++ glibc-2_9/nptl/init.c	2010-08-18 10:46:06.109098646 +0200

@@ -236,7 +236,7 @@

 /* This can be set by the debugger before initialization is complete.  */

-static bool __nptl_initial_report_events;

+static bool __nptl_initial_report_events __attribute_used__;

 void

 __pthread_initialize_minimal_internal (void)