Do not warn on multiple custom commands for the same output (#14446)

The VS 8 generator triggers the warning, and it is possible there are
valid use cases with different rules to generate the same output going
into different targets with different MAIN_DEPENDENCY options.  Leave
a TODO comment for warning in the future.
Merge topic 'fix-duplicate-custom-commands' into next

3e888ca Do not warn on multiple custom commands for the same output (#14446)