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

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

Comienza con la lista de comprobación del issue y el pull request de spatial-algorithms referenciado. Verifica los requisitos de la toolchain de Linux y OS X, los flags, las variables de entorno y las indicaciones sobre errores del linker; la documentación estará completa cuando cubra cada detalle de configuración y resolución de problemas indicado.