package/gd: Switch from libvpx to webp

Since bumping libvpx to 1.4.0
http://git.buildroot.net/buildroot/commit/package/libvpx?id=7d9a0c4d3960bb470e993494ac350b1415b72442

building gd was broken.
This patch adds some upstream commits which switch the dependency from libvpx to webp.

Fixes
http://autobuild.buildroot.net/results/046/046dd505feb5e92bdee3d0993366be162da1223a/
http://autobuild.buildroot.net/results/617/61739df0009015451ba78a7ca335dcc0d0dedcc8/
http://autobuild.buildroot.net/results/526/526550e73581a91427b394d566d3389554ee90ed/
http://autobuild.buildroot.net/results/b89/b89d7e3a1fc9403984bcd6462b8fd8d1196f2095/
http://autobuild.buildroot.net/results/dfe/dfed2b62aad83cc960ba3c93b7f0a994f18ad22a/
http://autobuild.buildroot.net/results/a91/a919d2bcbbd573e7a5556fbcdea053d4d451dd50/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>