nettle: add patch to support out-of-tree builds

Signed-off-by: Stephan Raue <stephan@openelec.tv>
nettle: set CC_FOR_BUILD

Signed-off-by: Stephan Raue <stephan@openelec.tv>
nettle: add patch to fix pkgconfig file, so gnutls is finding all nettle headers

Signed-off-by: Stephan Raue <stephan@openelec.tv>