build: enable `-Wall` and `-Wextra` options

CMakeLists.txt

@@ -1,6 +1,12 @@
 cmake_minimum_required(VERSION 3.0)
-set(CMAKE_CXX_STANDARD 14)
+
 project(klotski)
+set(CMAKE_CXX_STANDARD 14)
+
+if(NOT CMAKE_BUILD_TYPE)
+    set(CMAKE_BUILD_TYPE Release)
+endif()
+
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra")
 
 add_subdirectory(src)
-#add_subdirectory(klotski)