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 -dur strace-4.5.15.orig/signal.c strace-4.5.15/signal.c

 --- strace-4.5.15.orig/signal.c	2007-01-11 23:08:38.000000000 +0100

 +++ strace-4.5.15/signal.c	2007-07-14 19:24:43.000000000 +0200

 @@ -1440,7 +1440,7 @@

  	  	tcp->u_rval = tcp->u_error = 0;

  		if(tcp->u_arg[0] == 0)

  		  	return 0;

 -		tcp->auxstr = sprintsigmask("mask now ", tcp->u_arg[1]);

 +		tcp->auxstr = sprintsigmask("mask now ", tcp->u_arg[1], 0);

  		return RVAL_NONE | RVAL_STR;

  	}

  	return 0;