The same VARNAME was used both with and without an & prefix.
This can be confusing. Also, if you avoid this then you will
get a warning iff you accidentally leave off a needed &.
+ The generation of this warning depends on scanning your
+ makefile for things that look like variable references, which
+ subdirmk does not do completely perfectly. Exciting make
+ syntax may evade this warning, or require suppressions.
(You can suppress this warning for a particular VARNAME with
the &:local+global directive.)