package/pkg-autotools: don't use config.site when cross-compiling

On fedora 20 64bits host, the file /usr/share/config.site contains
a fix for installing libraries into /lib/lib64 on 64bits systems
that redefine libdir in the generated Makefile

For safety and avoid the bug #7262 [1], disable loading this file
when running the configure script for the target and the host.

Note: configure scripts generated with autoconf < 2.65 will source
the /dev/null and print this line:
"configure: loading site script /dev/null"

[1]: https://bugs.busybox.net/show_bug.cgi?id=7262



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>