Configure CMake itself with policies through CMake 3.15

In particular, set `CMP0091` to `NEW` to enable the MSVC runtime library
abstraction so it can be set via `CMAKE_MSVC_RUNTIME_LIBRARY` in the
cache.

CMakeLists.txt

@@ -1,13 +1,24 @@
 # 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...3.14 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.1...3.15 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)
 unset(CMAKE_USER_MAKE_RULES_OVERRIDE_CXX)
 unset(CMAKE_USER_MAKE_RULES_OVERRIDE_C)
 
+# FIXME: This block should go away after a transition period.
+if(MSVC AND NOT CMAKE_VERSION VERSION_LESS 3.15)
+  # Filter out MSVC runtime library flags that may have come from
+  # the cache of an existing build tree or from scripts.
+  foreach(l C CXX)
+    foreach(c DEBUG MINSIZEREL RELEASE RELWITHDEBINFO)
+      string(REGEX REPLACE "[-/]M[DT]d?( |$)" "" "CMAKE_${l}_FLAGS_${c}" "${CMAKE_${l}_FLAGS_${c}}")
+    endforeach()
+  endforeach()
+endif()
+
 # Make sure we can find internal find_package modules only used for
 # building CMake and not for shipping externally
 list(INSERT CMAKE_MODULE_PATH 0 ${CMake_SOURCE_DIR}/Source/Modules)

Utilities/Doxygen/CMakeLists.txt

@@ -3,7 +3,7 @@
 
 if(NOT CMake_SOURCE_DIR)
   set(CMakeDeveloperReference_STANDALONE 1)
-  cmake_minimum_required(VERSION 3.1...3.14 FATAL_ERROR)
+  cmake_minimum_required(VERSION 3.1...3.15 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/Release/WiX/CustomAction/CMakeLists.txt

@@ -1,9 +1,15 @@
-foreach(CONFIG DEBUG MINSIZEREL RELEASE RELWITHDEBINFO)
-  string(REPLACE "/MD" "/MT"
-    "CMAKE_CXX_FLAGS_${CONFIG}"
-    "${CMAKE_CXX_FLAGS_${CONFIG}}"
-  )
-endforeach()
+if(MSVC)
+  if(NOT CMAKE_VERSION VERSION_LESS 3.15)
+    set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
+  else()
+    foreach(CONFIG DEBUG MINSIZEREL RELEASE RELWITHDEBINFO)
+      string(REPLACE "/MD" "/MT"
+        "CMAKE_CXX_FLAGS_${CONFIG}"
+        "${CMAKE_CXX_FLAGS_${CONFIG}}"
+        )
+    endforeach()
+  endif()
+endif()
 
 add_library(CMakeWiXCustomActions MODULE
   detect_nsis_overwrite.cpp

Utilities/Sphinx/CMakeLists.txt

@@ -3,7 +3,7 @@
 
 if(NOT CMake_SOURCE_DIR)
   set(CMakeHelp_STANDALONE 1)
-  cmake_minimum_required(VERSION 3.1...3.14 FATAL_ERROR)
+  cmake_minimum_required(VERSION 3.1...3.15 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)