CMake: Honor _ROOT Env Hints

[ax3l]

CMake 3.12.0+ honor <Package>_ROOT environment hints which are often set on HPC systems. Previously, it was only looking for <Package>_DIR paths in find_package calls.

This new policy is useful since HPC systems usually set _DIR, _ROOT or expand the CMAKE_PREFIX_PATH. Therefore we want to use it as soon as it is available.

On systems where those env vars are set, e.g. Hypnos, this also throws a warning if the default (OLD) policy is used with CMake 3.12.4 or newer.

References:

• https://cmake.org/cmake/help/v3.12/policy/CMP0074.html
• https://github.com/ComputationalRadiationPhysics/picongpu/pull/2891

[psychocoderHPC]

@ax3l is this policy than automatically set in the generated cmake file out of SplashConfig.cmake.in?

[ax3l]

@psychocoderHPC I think yes, but didn't verify. At least I didn't see anything like this in ADIOS2, which is my usual reference for modern CMake ;-)

https://github.com/ornladios/ADIOS2/search?q=CMP0074&unscoped_q=CMP0074

[ax3l]

I fixed the CI in #282, so this should run successfully now.

[ax3l]

@psychocoderHPC ready for merge :)

[ax3l]

ping @psychocoderHPC