Fix compilation error on Ubuntu 18.04 (ROS Melodic)

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8.3)
+cmake_minimum_required(VERSION 3.1.3)
 project(loam_velodyne)
 
 find_package(catkin REQUIRED COMPONENTS
@@ -29,7 +29,8 @@ catkin_package(
 
 ## Compile as C++14, supported in ROS Kinetic and newer
 # set_property(TARGET invz_player PROPERTY CXX_STANDARD 17)
-add_compile_options(-std=c++14)
+set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 add_definitions( -march=native )