CMake: Support <PackageName>_ROOT

[reddwarf69]

I am no expert in all the policies, but I have seen no issues when using it with cmake 3.21.2.
My main user-case is getting fmt_ROOT to work (CMP0074).

[gabime]

The lines

if(POLICY CMP0077)
    cmake_policy(SET CMP0077 NEW) # option() honors variables already set
endif()

were added in pr #1946. Do you think it was wrong ?

[reddwarf69]

No, it wasn't wrong. But after using cmake_minimum_required(VERSION 3.10...3.21) it becomes redundant. With it every cmake version knowing about CMP0077 will use CMP0077 NEW. In fact every cmake version <= 3.21 knowing about any CMPXXXX will use CMPXXXX NEW.

[gabime]

Thanks. Merged