CMake: set policy CMP0074 to NEW to fix custom package paths for CMake 3.12 and above

[davidebeatrici]

From https://cmake.org/cmake/help/git-stage/policy/CMP0074.html:

find_package() uses <PackageName>_ROOT variables.

In CMake 3.12 and above the find_package(<PackageName>) command now searches prefixes specified by the <PackageName>_ROOT CMake variable and the <PackageName>_ROOT environment variable. Package roots are maintained as a stack so nested calls to all find_* commands inside find modules also search the roots as prefixes. This policy provides compatibility with projects that have not been updated to avoid using <PackageName>_ROOT variables for other purposes.

The OLD behavior for this policy is to ignore <PackageName>_ROOT variables. The NEW behavior for this policy is to use <PackageName>_ROOT variables.

This policy was introduced in CMake version 3.12. CMake version 3.12.20180904-gb694d warns when the policy is not set and uses OLD behavior. Use the cmake_policy() command to set it to OLD or NEW explicitly.