This is the big change !!!!
Instead of handling target as primary object and prepare/build targets,
we are handling build steps.
A build step may be a source (preparation) or a build (of the source).
Actualy, a step is a tuple (context, Builder or Source).
The context define the context of the step. It can be :
- 'source', for a Source step
- 'neutral' or the name of a platform for Build step.
Target becomes a "Class only" class.
Introduce also a "NeutralEnv", a build environment independent of the
targeted platform. All `Source` now build using the neutralEnv.
Most of toolchains are also using neutralEnv except android_ndk who is
specific to a platform.
As toolchain are neutral, platform specific environment variables are now
set by the platformInfo directly instead of the toolchain.
Matthieu Gautier