Debian Testing compatibility with CMake < 3.5 has been removed from CMake

[GermanAizek]

CMake Error at external/SPIRV-Tools/CMakeLists.txt:9 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.

CMake minimum version is deprecated, it is necessary to increase minimum version for assembly on modern version Debian distro.