Require CMake 3.12+ for using OpenMP backend

CMake/VTKmDeviceAdapters.cmake

@@ -86,7 +86,7 @@ endif()
 
 
 if(VTKm_ENABLE_OPENMP AND NOT TARGET vtkm::openmp)
-  cmake_minimum_required(VERSION 3.9...3.15 FATAL_ERROR)
+  cmake_minimum_required(VERSION 3.12...3.15 FATAL_ERROR)
   find_package(OpenMP 4.0 REQUIRED COMPONENTS CXX QUIET)
 
   add_library(vtkm::openmp INTERFACE IMPORTED GLOBAL)

CMakeLists.txt

@@ -250,7 +250,6 @@ if(NOT VTKm_INSTALL_ONLY_LIBRARIES)
       ${VTKm_SOURCE_DIR}/CMake/FindTBB.cmake
       ${VTKm_SOURCE_DIR}/CMake/FindMPI.cmake
       ${VTKm_SOURCE_DIR}/CMake/FindOpenGL.cmake
-      ${VTKm_SOURCE_DIR}/CMake/FindOpenMP.cmake
     DESTINATION ${VTKm_INSTALL_CMAKE_MODULE_DIR}
     )
 

README.md

@@ -65,8 +65,8 @@ VTK-m Requires:
       + Intel 17.0.4+
   + [CMake](http://www.cmake.org/download/)
       + CMake 3.8+
-      + CMake 3.9+  (for OpenMP support)
       + CMake 3.11+ (for Visual Studio generator)
+      + CMake 3.12+ (for OpenMP support)
       + CMake 3.13+ (for CUDA support)
 
 Optional dependencies are: