Declare support for CMake versions through 3.12 for own build

CMake 3.12 introduced a `...<max>` syntax in the version given to
`cmake_minimum_required` to automatically set policies to NEW up
to that version.  Use it to avoid listing policies explicitly.

The syntax is compatible with older versions of CMake such that they use
the extended version string for the `CMAKE_MINIMUM_REQUIRED_VERSION`
variable (which we don't use) but otherwise ignore it.

CMakeLists.txt

@@ -1,7 +1,7 @@
 # Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
 # file Copyright.txt or https://cmake.org/licensing for details.
 
-cmake_minimum_required(VERSION 3.1 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.1...3.12 FATAL_ERROR)
 set(CMAKE_USER_MAKE_RULES_OVERRIDE_C ${CMAKE_CURRENT_SOURCE_DIR}/Source/Modules/OverrideC.cmake)
 set(CMAKE_USER_MAKE_RULES_OVERRIDE_CXX ${CMAKE_CURRENT_SOURCE_DIR}/Source/Modules/OverrideCXX.cmake)
 project(CMake)

Utilities/Doxygen/CMakeLists.txt

@@ -3,7 +3,7 @@
 
 if(NOT CMake_SOURCE_DIR)
   set(CMakeDeveloperReference_STANDALONE 1)
-  cmake_minimum_required(VERSION 3.1 FATAL_ERROR)
+  cmake_minimum_required(VERSION 3.1...3.12 FATAL_ERROR)
   get_filename_component(tmp "${CMAKE_CURRENT_SOURCE_DIR}" PATH)
   get_filename_component(CMake_SOURCE_DIR "${tmp}" PATH)
   include(${CMake_SOURCE_DIR}/Modules/CTestUseLaunchers.cmake)

Utilities/Sphinx/CMakeLists.txt

@@ -3,7 +3,7 @@
 
 if(NOT CMake_SOURCE_DIR)
   set(CMakeHelp_STANDALONE 1)
-  cmake_minimum_required(VERSION 3.1 FATAL_ERROR)
+  cmake_minimum_required(VERSION 3.1...3.12 FATAL_ERROR)
   get_filename_component(tmp "${CMAKE_CURRENT_SOURCE_DIR}" PATH)
   get_filename_component(CMake_SOURCE_DIR "${tmp}" PATH)
   include(${CMake_SOURCE_DIR}/Modules/CTestUseLaunchers.cmake)