Remove explicit setting of CM0074 policy to NEW

[shrijitsingh99]

Expanding upon #2671
Couldn't quite understand why this was needed:

pcl/PCLConfig.cmake.in

Line 20 in 8bcde6f

# TODO: update *_ROOT variables to be PCL_*_ROOT or equivalent.

[kunaltyagi]

There's another PR working on Boost. Can't link it now

[shrijitsingh99]

There's another PR working on Boost. Can't link it now

I had mistakenly modified the Boost dependency. Removed it.

[kunaltyagi]

Couldn't quite understand why this was needed:

CMake considers <module>_ROOT variables to be used to find the CMake module called <module>. By hiding them with a PCL_ prefix,

1. we can prevent name clashes since we can assume nobody but PCL will make a module called PCL_surface
2. BUT other people can release modules called surface to find which
3. CMake will search for surface_ROOT and due to our 'new' naming, there will not be any confusions between surface_ROOT and PCL_surface_ROOT

[SergioRAgostinho]

Couldn't quite understand why this was needed:

Recent versions of cmake we're at some point spewing the warning claiming that policy was not set. This comment clarifies under which situations it is emitted.

If you're using CMake >= 3.12 and no warning related the policy is being emitted, then it might mean that it is no longer required. Try this both for pcl and a downstream project with a cmake_minimum_required < 3.12, while using a CMake version >= 3.12.

[shrijitsingh99]

2. BUT other people can release modules called `surface` to find which

3. CMake will search for `surface_ROOT` and due to our 'new' naming, there will not be any confusions between `surface_ROOT` and `PCL_surface_ROOT`

Still slightly confused. We use BOOST_ROOT to help CMake discover the Boost module. in all_inone_installermode. So why create something likePCL_BOOST_ROOT`, what would be its purpose?

If you're using CMake >= 3.12 and no warning related the policy is being emitted, then it might mean that it is no longer required. Try this both for pcl and a downstream project with a cmake_minimum_required < 3.12, while using a CMake version >= 3.12.

Will test and report back.

[shrijitsingh99]

I checked it out, didn't notice any CMake warnings on setting minimum version to 3.0

[kunaltyagi]

@SergioRAgostinho I'm not comfortable merging PR with CMake changes. PTAL

[SergioRAgostinho]

I checked it out, didn't notice any CMake warnings on setting minimum version to 3.0

We might have removed the previous usage of *_ROOT in most finder scripts. I still see them being set in PCLConfig for PCL_ALL_IN_ONE_INSTALLER code. I would be more comfortable unsetting the policy once all situations in which a *_ROOT variable is set are removed from the code base.

[kunaltyagi]

Maybe @larshg can help review the CMake code so there's more confidence in merging this

[larshg]

As I can see it, Package_ROOT variables are still used in various find scripts. And the policy is first available from 3.12. I couldn't find any information if it has been removed in later Cmake versions, but I don't get any warnings not setting the policy, running Cmake 3.19.1.
But most dependencies are also found with config files now or with find_package.

However, I don't see any reason not to keep it as it just allow CMake to look in package_ROOT directories if they are defined.
We could however remove all the XX_ROOT variables, but first when CMake 3.12 are required. If we remove the paths now, all old CMake versions that doesn't know the policy, would go with the old behavior and won't automatically look in package_ROOT folders and eventually lead to dependencies not being found.

I would close this one for now.

[kunaltyagi]

Will reopen if there's interest by original author post 3.12 requirement