Build configuration for the compiler used for bootstrapping. If not set, then use the default build configuration. Used when fromSource is set.
Whether to download a pre-built D version, or build one from source. If set, then build from source according to the value of ver,
Version specification. When building from source, syntax can be defined by outer application (see parseSpec method); When the bootstrapping compiler is not built from source, it is understood as a version number, such as "v2.070.2", which also doubles as a tag name. By default (when set to null), an appropriate version is selected automatically.
How to build DMD versions written in D. We can either download a pre-built binary DMD package, or build an earlier version from source (e.g. starting with the last C++-only version.)