Option for switching static runtime link policy with MSVC

cmake/CMakeLists.txt

@@ -8,6 +8,7 @@ project(protobuf C CXX)
 option(protobuf_VERBOSE "Enable for verbose output" OFF)
 option(BUILD_TESTING "Build tests" ON)
 option(BUILD_SHARED_LIBS "Build Shared Libraries" OFF)
+option(protobuf_MSVC_STATIC_RUNTIME "Link static runtime libraries" ON)
 if (MSVC)
   set(ZLIB_DEFAULT OFF)
 else (MSVC)
@@ -92,13 +93,15 @@ if (MSVC)
     # 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
-    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)
+    if (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 (protobuf_MSVC_STATIC_RUNTIME)
   endif (BUILD_SHARED_LIBS)
   add_definitions(/wd4244 /wd4267 /wd4018 /wd4355 /wd4800 /wd4251 /wd4996 /wd4146 /wd4305)
 endif (MSVC)