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)

http://git.frugalware.org/gitweb/gitweb.cgi?p=frugalware-current.git;a=blob_plain;f=source/base/glibc/make-3.82-fix.patch;hb=8217c32ecc2e14962847ba3d8a272eb64a3dba4f

--- glibc-2.10.1.OLD/manual/Makefile

+++ glibc-2.10.1.NEW/manual/Makefile

@@ -232,7 +232,9 @@

 .PHONY: stubs

 stubs: $(objpfx)stubs

 endif

-$(objpfx)stubs ../po/manual.pot $(objpfx)stamp%:

+$(objpfx)stubs ../po/manual.pot:

+	touch $@

+$(objpfx)stamp%:

 	$(make-target-directory)

 	touch $@