message: specify c++ standard via CMake variable
diff: @@ -11,9 +11,11 @@ endif()
  PROJECT(iroha C CXX)
  +SET(CMAKE_CXX_STANDARD 14)
  +
  SET(CMAKE_POSITION_INDEPENDENT_CODE TRUE)
  if (NOT MSVC)
  - SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14 -Wall -fdiagnostics-color=always")
  + SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -fdiagnostics-color=always")
  SET(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -O3 -Wno-error=deprecated-declarations")
  SET(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -g -Wextra -Wno-unused-parameter -Wno-deprecated-declarations -O0")
  endif()
  