fix CMAKE_CXX_FLAGS

Author: toge

CMakeLists.txt

@@ -37,7 +37,7 @@ if (${CMAKE_CXX_COMPILER_ID} MATCHES "Clang" OR
         ${CMAKE_CXX_COMPILER_ID} STREQUAL "GNU")
     set(OPT "-O3")
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -g -Wall -Wextra -pedantic -std=c99 -O3 ${OPT}")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -g -Wall -Wextra -Woverloaded-virtual -pedantic -std=c++11 -fPIC ${OPT}")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -g -Wall -Wextra -Woverloaded-virtual -pedantic -std=c++17 -fPIC ${OPT}")
 else()
     # TODO: Windows support.
     message(FATAL_ERROR "Compiler ${CMAKE_CXX_COMPILER_ID} not supported")