gd: disable webp/vpx support

The recent update to libvpx 1.4.x broke gd. Upstream has replaced libvpx with libwebp, so fixing it isn't worth it.

If webp support is really needed, backport https://bitbucket.org/libgd/gd-libgd/commits/a79232c5fa69

 and add a PACKAGECONFIG for it.

Signed-off-by: Koen Kooi <koen.kooi@linaro.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>