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 -ru gdb-6.8.orig/bfd/sysdep.h gdb-6.8/bfd/sysdep.h

--- gdb-6.8.orig/bfd/sysdep.h	2007-07-03 16:26:42.000000000 +0200

+++ gdb-6.8/bfd/sysdep.h	2011-08-21 13:33:47.000000000 +0200

@@ -131,7 +131,7 @@

 extern PTR realloc ();

 #endif

-#if !HAVE_DECL_STPCPY

+#if !HAVE_DECL_STPCPY && !defined(stpcpy)

 extern char *stpcpy (char *__dest, const char *__src);

 #endif