Use cache variables to customize

Use cache variables in CMakeLists.txt to customize build settings.

Author: hotwatermorning

CMakeLists.txt

@@ -8,10 +8,10 @@ set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/CMake;${CMAKE_MODULE_PATH}")
 
 project(ixwebsocket LANGUAGES C CXX VERSION 11.4.4)
 
-set (CMAKE_CXX_STANDARD 11)
-set (CXX_STANDARD_REQUIRED ON)
-set (CMAKE_CXX_EXTENSIONS OFF)
-set (CMAKE_EXPORT_COMPILE_COMMANDS yes)
+set (CMAKE_CXX_STANDARD 11 CACHE STRING "C++ Standard Version")
+set (CMAKE_CXX_STANDARD_REQUIRED ON BOOL "C++ Standard Required")
+set (CMAKE_CXX_EXTENSIONS OFF BOOL "C++ Extensions")
+set (CMAKE_EXPORT_COMPILE_COMMANDS yes BOOL "Export Compile Commands")
 
 option (BUILD_DEMO OFF)