Revert "Suppress CMake policy warning"

This reverts commit 4d0b0acb58ceb3a2337d80259f33fd3ce44d0f4b.

This doesn't seem to work with old versions of CMAke:

CMake Error at CMakeLists.txt:4 (cmake_policy):
  Policy "CMP0043" is not known to this version of CMake.