Remove policy push/pop from "PCLConfig.cmake" file

[SergioRAgostinho]

These are not required because cmake automatically creates a new stack entry when find_package() is invoked.

Closes #2726, fixes #3062.

[taketwo]

According to your initial comment in the issue (#2726 (comment)):

From what I can tell, if we simply remove the push/pop policy our PCLConfig.cmake will start producing warnings in consumer projects.

This PR does remove push/pop and also sets cmake_policy(VERSION 3.5). Do I understand right that the latter is the key to avoid warnings?

[SergioRAgostinho]

This PR does remove push/pop and also sets cmake_policy(VERSION 3.5). Do I understand right that the latter is the key to avoid warnings?

Yes and no. Warnings are avoided, when policies are explicitly set. cmake_policy(VERSION 3.5) does exactly this without us having to manually set each policy individually. Since we are already supporting CMP0074 which appeared after CMake 3.5, this one needs to be set explicitly.

We started using push/pop because we thought that without it, our config file would be setting project wide policies for our consumers project. However, I later discovered that calls to find_package() create a new entry policy stack. This means that whatever policies are set inside find package/config scripts are restricted to that context and do not propagate to the rest of the project. It also meant that explicit calls to push/pop are not required.

[taketwo]

Thanks for the explanation!