message: Require flex 2.6 to make it conformant with C++17
diff: @@ -194,7 +194,7 @@ find_package(BISON 3.0 REQUIRED)
  )
  endif()
  -find_package(FLEX 2.5 REQUIRED)
  +find_package(FLEX 2.6 REQUIRED)
  if (FLEX_FOUND)
  set(FLEX_INPUT src/lex.ll)
  set(FLEX_OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}/lex.cpp)
  