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/configure.ac strace-4.5.15/configure.ac

--- strace-4.5.15.orig/configure.ac	2007-07-14 19:25:25.000000000 +0200

+++ strace-4.5.15/configure.ac	2007-07-14 19:27:35.000000000 +0200

@@ -91,14 +91,14 @@

 	arch=hppa

 	AC_DEFINE([HPPA], 1, [Define for the HPPA architecture.])

 	;;

-sh)

-	arch=sh

-	AC_DEFINE([SH], 1, [Define for the SH architecture.])

-	;;

 sh64)

 	arch=sh64

 	AC_DEFINE([SH64], 1, [Define for the SH64 architecture.])

 	;;

+sh*)

+	arch=sh

+	AC_DEFINE([SH], 1, [Define for the SH architecture.])

+	;;

 x86?64*)

 	arch=x86_64

 	AC_DEFINE([X86_64], 1, [Define for the AMD x86-64 architecture.])