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>

Index: ltrace-0.5.3/configure

===================================================================

--- ltrace-0.5.3/configure

+++ ltrace-0.5.3/configure

@@ -78,7 +78,10 @@ fi

 rm -f conftest.c a.out

 CFLAGS='-g -O2'

-LIBS='-lelf -lsupc++ -liberty '

+LIBS='-lelf'

+[ -n "$HAVE_LIBSUPC__" ] && LIBS="$LIBS -lsupc++"

+[ -n "$HAVE_LIBIBERTY" ] && LIBS="$LIBS -liberty"

+

 INSTALL='install -c'

 iquote='-iquote '

 iquoteend=''