cmake: support MSVC_RUNTIME_LIBRARY property

When using a new enough CMake (3.15+) prefer to use the
`MSVC_RUNTIME_LIBRARY` property on targets to select the runtime library
variant.  This property is automatically set to the value specified by
`CMAKE_MSVC_RUNTIME_LIBRARY`.  This property requires that the CMake
Policy 91 is set to new (see CMP0091).

cmake/CMakeLists.txt

@@ -15,6 +15,10 @@ endif ()
 if(POLICY CMP0048)
   cmake_policy(SET CMP0048 NEW)
 endif()
+# MSVC runtime library flags are selected by an abstraction.
+if(POLICY CMP0091)
+  cmake_policy(SET CMP0091 NEW)
+endif()
 
 # Project
 project(protobuf C CXX)
@@ -176,20 +180,28 @@ if (protobuf_BUILD_SHARED_LIBS)
   set(protobuf_SHARED_OR_STATIC "SHARED")
 else (protobuf_BUILD_SHARED_LIBS)
   set(protobuf_SHARED_OR_STATIC "STATIC")
-  # In case we are building static libraries, link also the runtime library statically
-  # so that MSVCR*.DLL is not required at runtime.
-  # https://msdn.microsoft.com/en-us/library/2kzt1wy3.aspx
-  # This is achieved by replacing msvc option /MD with /MT and /MDd with /MTd
-  # http://www.cmake.org/Wiki/CMake_FAQ#How_can_I_build_my_MSVC_application_with_a_static_runtime.3F
-  if (MSVC AND protobuf_MSVC_STATIC_RUNTIME)
-    foreach(flag_var
-        CMAKE_CXX_FLAGS CMAKE_CXX_FLAGS_DEBUG CMAKE_CXX_FLAGS_RELEASE
-        CMAKE_CXX_FLAGS_MINSIZEREL CMAKE_CXX_FLAGS_RELWITHDEBINFO)
-      if(${flag_var} MATCHES "/MD")
-        string(REGEX REPLACE "/MD" "/MT" ${flag_var} "${${flag_var}}")
-      endif(${flag_var} MATCHES "/MD")
-    endforeach(flag_var)
-  endif (MSVC AND protobuf_MSVC_STATIC_RUNTIME)
+  # The CMAKE_<LANG>_FLAGS(_<BUILD_TYPE>)? is meant to be user controlled.
+  # Prior to CMake 3.15, the MSVC runtime library was pushed into the same flags
+  # making programmatic control difficult.  Prefer the functionality in newer
+  # CMake versions when available.
+  if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.15)
+    set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreaded$<$<CONFIG:Debug>:Debug>)
+  else()
+    # In case we are building static libraries, link also the runtime library statically
+    # so that MSVCR*.DLL is not required at runtime.
+    # https://msdn.microsoft.com/en-us/library/2kzt1wy3.aspx
+    # This is achieved by replacing msvc option /MD with /MT and /MDd with /MTd
+    # http://www.cmake.org/Wiki/CMake_FAQ#How_can_I_build_my_MSVC_application_with_a_static_runtime.3F
+    if (MSVC AND protobuf_MSVC_STATIC_RUNTIME)
+      foreach(flag_var
+          CMAKE_CXX_FLAGS CMAKE_CXX_FLAGS_DEBUG CMAKE_CXX_FLAGS_RELEASE
+          CMAKE_CXX_FLAGS_MINSIZEREL CMAKE_CXX_FLAGS_RELWITHDEBINFO)
+        if(${flag_var} MATCHES "/MD")
+          string(REGEX REPLACE "/MD" "/MT" ${flag_var} "${${flag_var}}")
+        endif(${flag_var} MATCHES "/MD")
+      endforeach(flag_var)
+    endif (MSVC AND protobuf_MSVC_STATIC_RUNTIME)
+  endif()
 endif (protobuf_BUILD_SHARED_LIBS)
 
 if (MSVC)