[cmake-developers] Severe regression caused by #14972 fixes

[Adam Strzelecki]

> This will not be done before the freeze for 3.1 on Thursday.
> Reverting 7243c951 will resolve the problem for ReactOS in
> out-of-source builds.  So, we either revert that or hope Ninja can
> be fixed to deal with the large dependency lists w/out crashing.

In meantime I've pushed stage/cmp0055-disable-ninja-side-effects that make CMake warn about this compatibility layer when such phony rules are about to be emitted.

Setting policy to NEW or making all deps to be explicit outputs disables warning, setting to OLD removed warning keeping phony rules.

IHMO this is win-win for everybody, since it will emit warnings once someone installs 3.1 and possibly relies on side-effect.

One thing I am not sure about is if the CMP0055.rst provided is clear enough for the reader.

--Adam