[Cmake] Subdirectory dependencies

[Amitha Perera]

Hi all

Recently, someone made each subdirectory depend on the previous one,
in order to solve some multi-threaded issues. However, it has an
unfortunate side effect in that even when there are no dependencies in
the subdirectories, one subdirectory failing to build means the entire
build process halts. A solution may be to allow explicit dependency
information in the SUBDIR command, with the default behaviour as now:
   SUBDIR( a )
   SUBDIR( b )
   SUBDIR( c )
    ->   b depends on a, and c on b (and hence on a).
   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.

Comments?

Amitha.