Custom command dependencies are followed for each target's source files
and add their transitive closure to the corresponding target.  This
means that when a custom command in one target has a dependency on a
custom command in another target, both will appear in the dependent
target's sources.  For the Makefile, VS IDE, and Xcode generators this
is not a problem because each target gets its own independent build
system that is evaluated in target dependency order.  By the time the
dependent target is built the custom command that belongs to one of its
dependencies will already have been brought up to date.

For the Ninja generator we need to generate a monolithic build system
covering all targets so we can have only one copy of a custom command.
This means that we need to reconcile the target-level ordering
dependencies from its appearance in multiple targets to include only the
least-dependent common set.  This is done by computing the se...