message: Fix CMake OS dependent default options.
  The defaults were setting local variables, while the "option" command
  uses the values from the CMake cache. This made the CI not build the
  D3D12 or Metal backend on their platforms.
diff: @@ -22,14 +22,16 @@ project(nxt C CXX)
  option(NXT_USE_WERROR "Treat warnings as error (useful for CI)" 0)
  # Default values for the backend-enabling options
  +set(ENABLE_D3D12 OFF)
  +set(ENABLE_METAL OFF)
  if (WIN32)
  - set(NXT_ENABLE_D3D12 ON)
  + set(ENABLE_D3D12 ON)
  elseif(APPLE)
  - set(NXT_ENABLE_METAL ON)
  + set(ENABLE_METAL ON)
  endif()
  -option(NXT_ENABLE_D3D12 "Enable compilation of the D3D12 backend" OFF)
  -option(NXT_ENABLE_METAL "Enable compilation of the Metal backend" OFF)
  +option(NXT_ENABLE_D3D12 "Enable compilation of the D3D12 backend" ${ENABLE_D3D12})
  +option(NXT_ENABLE_METAL "Enable compilation of the Metal backend" ${ENABLE_METAL})
  option(NXT_ENABLE_NULL "Enable compilation of the Null backend" ON)
  option(NXT_ENABLE_OPENGL "Enable compilation of the OpenGL backend" ON)
  option(NXT_ENABLE_VULKAN "Enable compilation of the Vulkan backend" OFF)
  