Add toolchain in all the recipes

[pradal]

To support ABI compatibiliy between all the recipes, I propose to use the conda-forge toolchain.
The toolchain defines all the flags (CXXFLAGS, ...), the compiler, ... that is the same for all the packages we build.

conda install toolchain -c conda-forge
See https://github.com/conda-forge/toolchain-feedstock

[pradal]

Another option is to use the Anaconda compilers.
Two solutions can be discussed:

1. We build a package compiler_toolchain that embeds the latest C, C++ and Fortran compilers available. All binary openalea packages will then include as their dependency compiler_toolchain.

This package will contain something like :

requirements:
  build:
    - {{ compiler('cxx') }}

In this case, to share both the compiler and to share exact package version, we need to share a conda_build_config.yaml

2. The second solution is, to depend on a compiler_toolchain package. This solution will of course rely on anaconda compiler, but will allow to not depend on conda_build_config.yaml

@pfernique @fredboudon @artzet-s

[pradal]

In conda-forge, they are looking at migrating from toolset to conda-build 3 and anaconda compilers:
conda-forge/conda-forge.github.io#460

[pradal]

I will migrate all the recipes on compilers