[CMAKE] `protobuf::libprotoc` and `protobuf::protoc` should be optional in legacy `protobuf-module.cmake` when crossing compiling.

cmake/protobuf-module.cmake.in

@@ -94,7 +94,7 @@ function(_protobuf_find_libraries name filename)
   elseif(${name}_LIBRARY)
     # Honor cache entry used by CMake 3.5 and lower.
     set(${name}_LIBRARIES "${${name}_LIBRARY}" PARENT_SCOPE)
-  else()
+  elseif(TARGET protobuf::lib${filename})
     get_target_property(${name}_LIBRARY_RELEASE protobuf::lib${filename}
       LOCATION_RELEASE)
     get_target_property(${name}_LIBRARY_RELWITHDEBINFO protobuf::lib${filename}
@@ -134,6 +134,7 @@ get_target_property(Protobuf_INCLUDE_DIRS protobuf::libprotobuf
   INTERFACE_INCLUDE_DIRECTORIES)
 
 # Set the protoc Executable
+if(NOT Protobuf_PROTOC_EXECUTABLE AND TARGET protobuf::protoc)
   get_target_property(Protobuf_PROTOC_EXECUTABLE protobuf::protoc
     IMPORTED_LOCATION_RELEASE)
   if(NOT EXISTS "${Protobuf_PROTOC_EXECUTABLE}")
@@ -152,6 +153,7 @@ if(NOT EXISTS "${Protobuf_PROTOC_EXECUTABLE}")
     get_target_property(Protobuf_PROTOC_EXECUTABLE protobuf::protoc
       IMPORTED_LOCATION_NOCONFIG)
   endif()
+endif()
 
 # Version info variable
 set(Protobuf_VERSION "@protobuf_VERSION@")