FindProtobuf: Fix usage of protobuf dynamic libraries on Windows

Define `PROTOBUF_USE_DLLS` on Windows when linking against dynamic
protobuf libraries so that we import symbols from them.

We use the condition `MSVC` to enable this because that is what
the upstream buildsystem uses.

Modules/FindProtobuf.cmake

@@ -526,6 +526,11 @@ if(Protobuf_INCLUDE_DIR)
               INTERFACE_COMPILE_FEATURES cxx_std_11
             )
           endif()
+          if (MSVC AND NOT Protobuf_USE_STATIC_LIBS)
+            set_property(TARGET protobuf::libprotobuf APPEND PROPERTY
+              INTERFACE_COMPILE_DEFINITIONS "PROTOBUF_USE_DLLS"
+            )
+          endif()
           if(UNIX AND TARGET Threads::Threads)
             set_property(TARGET protobuf::libprotobuf APPEND PROPERTY
                 INTERFACE_LINK_LIBRARIES Threads::Threads)
@@ -554,6 +559,11 @@ if(Protobuf_INCLUDE_DIR)
             set_target_properties(protobuf::libprotobuf-lite PROPERTIES
               IMPORTED_LOCATION_DEBUG "${Protobuf_LITE_LIBRARY_DEBUG}")
           endif()
+          if (MSVC AND NOT Protobuf_USE_STATIC_LIBS)
+            set_property(TARGET protobuf::libprotobuf-lite APPEND PROPERTY
+              INTERFACE_COMPILE_DEFINITIONS "PROTOBUF_USE_DLLS"
+            )
+          endif()
           if(UNIX AND TARGET Threads::Threads)
             set_property(TARGET protobuf::libprotobuf-lite APPEND PROPERTY
                 INTERFACE_LINK_LIBRARIES Threads::Threads)
@@ -587,6 +597,11 @@ if(Protobuf_INCLUDE_DIR)
               INTERFACE_COMPILE_FEATURES cxx_std_11
             )
           endif()
+          if (MSVC AND NOT Protobuf_USE_STATIC_LIBS)
+            set_property(TARGET protobuf::libprotoc APPEND PROPERTY
+              INTERFACE_COMPILE_DEFINITIONS "PROTOBUF_USE_DLLS"
+            )
+          endif()
           if(UNIX AND TARGET Threads::Threads)
             set_property(TARGET protobuf::libprotoc APPEND PROPERTY
                 INTERFACE_LINK_LIBRARIES Threads::Threads)