When crossbuilding e.g. from a Linux system it is not shipped as part of
bmake and will cause the build to fail. I am not sure if it needs to be
included by the toplevel makefile in order for ccache builds to work since
I haven't tested that yet on Linux/Mac.
Another way of solving this could be to required that cross os builds
always set -m $SRCDIR/share/mk but it seems to me like this small change
is simpler.