check new support for package_ROOT in CMake, in particular for BOOST

[KrisThielemans]

On Appveyor tests, we're now getting

CMake Warning (dev) at CMakeLists.txt:66 (find_package):
  Policy CMP0074 is not set: find_package uses <PackageName>_ROOT variables.
  Run "cmake --help-policy CMP0074" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.
  Environment variable Boost_ROOT is set to:
    C:\Libraries\boost_1_63_0
  For compatibility, CMake is ignoring the variable.
This warning is for project developers.  Use -Wno-dev to suppress it.
-- Boost version: 1.63.0

It did seem to work (there's an unrelated error in that log) though. We need to at least get rid of the warning.

Thisis from CMake 3.12, see https://cmake.org/cmake/help/git-stage/policy/CMP0074.html

[KrisThielemans]

Closed by #1140