[Cmake] Subdirectory dependencies

[Brad King]

> Recently, someone made each subdirectory depend on the previous one,
> in order to solve some multi-threaded issues.
[snip]
>    SUBDIR( a )
>    SUBDIR( b DEPENDS a )
>    SUBDIR( c DEPENDS a )
>     ->   b depends on a, c depends on a. If b fails, c can still
>          build. b and c could be built in parallel.
This had already been implemented as a "SUBDIR_DEPENDS" command, but I
recently removed it in favor of the automatic approach.  Since all the
projects we had been using it on didn't have compiler errors, we didn't
notice.

We'll look into it, but I'm guessing we'll have to put back the old
SUBDIR_DEPENDS functionality.

-Brad