Fix building with USECCACHE=1.

After 6300cbe5307056d9185d85fb4b9b22ab7d471fb9, building with `USECCACHE=1` was broken due to the alterations to `CC` and `CXX`, causing `$(BINARY)` to expand badly and ending up with `-m` rather than `-m64` in the compilation command.

This fixes it, although to be honest I'm not completely sure about this. Maybe some `Make` guru can comment on whether this is the proper way to prepend `ccache` to the recursively expanding `CC` and `CXX` variables.