message: Fix RANGES_CXX_STD handling in CMakeLists.txt
  ...by treating the value numerically only if the string is a sequence of digits.
diff: @@ -50,12 +50,14 @@ else()
  message(WARNING "[range-v3 warning]: unknown system ${CMAKE_SYSTEM_NAME} !")
  endif()
  +if(RANGES_CXX_STD MATCHES "^[0-9]+$")
  if(RANGES_CXX_COMPILER_MSVC AND RANGES_CXX_STD LESS 17)
  # MSVC is currently supported only in 17+ mode
  set(RANGES_CXX_STD 17)
  elseif(RANGES_CXX_STD LESS 14)
  set(RANGES_CXX_STD 14)
  endif()
  +endif()
  # Build type
  set(RANGES_DEBUG_BUILD FALSE)
  