Docs on enabling LTO (-flto) with clang++

Personne n'a encore pris cette issue.

Getting Link Time optimization working takes certain combinations of modern tools and build settings. It would be great to have some general docs on this so that LTO is demystified and more commonly used.

Doc should cover:

### Linux details

- upgraded binutils gold (can be installed via mason)
- at least clang++ 3.9.1 via mason
- llvm-ar and llvm-ranlib on path (and set in environment `AR=/path/to/llvm-ar` `RANLIB=/path/to/llvm-ranlib`)
- `-flto` in the CXXFLAGS

### OS X details
- Apple built in compiler or clang++ via mason
- `-flto` in the CXXFLAGS and LDFLAGS

#### Troubleshooting:

This error indicates that the linker does not understand the LTO format. This will happen on linux if you've not installed binutils from mason.

```
file.o: file not recognized: File format not recognized
clang-5.0: error: linker command failed with exit code 1 (use -v to see invocation)
```

Refs https://github.com/mapbox/spatial-algorithms/pull/10

Commencez par la liste de contrôle de l’issue et le pull request référencé de spatial-algorithms. Vérifiez les exigences de la toolchain pour Linux et OS X, les flags, les variables d’environnement et les indications concernant les erreurs du linker ; la documentation est terminée lorsqu’elle couvre chaque détail de configuration et de dépannage indiqué.

Rédigé par le modèle d'indexation à partir du texte de l'issue.