Can't we have something like what's [done](https://github.com/mozilla-b2g/platform_build/blob/master/core/combo/select.mk#L62) for ccache?  Set `$(combo_target)CC` to `$(realpath $(combo_target)CC)` in [select.mk](https://github.com/mozilla-b2g/platform_build/blob/master/core/combo/select.mk)?