community/opencv: upgrade to 4.7.0

since changing to clang to fix ppc builds a while ago, the LTO variable didn't do anything- with clang only THIN_LTO is available, but also needs lld, so i added that too. could remove it entirely if wanted too- but it works, so no harm like this.

also adds gst-base dep (otherwise it's seen as not found for all of gst) and libva support