Fix CMP0074 policy warning

[jcfr]

CMake Warning (dev) at /usr/share/cmake-3.13/Modules/FindPNG.cmake:51 (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.

  CMake variable ZLIB_ROOT is set to:

    /work/dcmjs-build/ZLIB-install

  For compatibility, CMake is ignoring the variable.
Call Stack (most recent call first):
  CMake/3rdparty.cmake:178 (find_package)
  CMake/dcmtkPrepare.cmake:426 (include)
  CMakeLists.txt:17 (include)
This warning is for project developers.  Use -Wno-dev to suppress it.