message: Add note that Mac requires a C++17 compiler
diff: @@ -37,6 +37,8 @@ To build with Xcode, use the cmake Xcode generator to create the Xcode project:
  cmake -DCMAKE_BUILD_TYPE=Debug -Bbuild -H. -GXcode
  ```
  +Building for Mac requires a C++17 compliant compiler i.e. the Xcode clang compiler.
  +
  ## Android
  First check that you have all of the [required dependencies](Dependencies.md#android).
  