build: use protobuf::protoc for the path to protoc compiler

and add it as a dependency for generating C++ bindings. simpler this
way. protobuf::protoc was added to FindProtobuf in CMake 3.10, and
our minimal required CMake version is 3.13, so we should be able to
have access to this variable as long as Protobuf is found.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

CMakeLists.txt

@@ -417,10 +417,11 @@ function (seastar_generate_protobuf)
   set (source_out ${args_OUT_DIR}/${in_file_name}.pb.cc)
 
   add_custom_command (
-    DEPENDS ${args_IN_FILE}
+    DEPENDS ${args_IN_FILE} protobuf::protoc
     OUTPUT ${header_out} ${source_out}
     COMMAND ${CMAKE_COMMAND} -E make_directory ${args_OUT_DIR}
-    COMMAND ${Protobuf_PROTOC_EXECUTABLE} --cpp_out=${args_OUT_DIR} -I${in_file_dir} ${args_IN_FILE})
+    COMMAND protobuf::protoc
+    ARGS --cpp_out=${args_OUT_DIR} -I${in_file_dir} ${args_IN_FILE})
 
   add_custom_target (${args_TARGET}
     DEPENDS