Downstream consumers of libx11 shouldn't have to know and account for libx11's headers/pkg-config files referencing xorgproto. A libx11-devel package would depend on xorgproto. Since there's no separate -devel package, the dependency stays with the regular libx11 package.
You already called (a) the pragmatic approach and it seems like a no-brainer at 1.5 MiB installed size. If we go with (c), it might be worthwhile to mass edit packages in /trunk to add the build dep. Doing that manually for hundreds of packages is a huge time sink and quite annoying during rebuilds.
