Dynamic-by-default was a mechanism to automatically select the -dynamic way for some targets.
It was implemented in a convoluted way: it was defined as a flavour
option, hence it couldn't be passed as a global settings (which are
configure before considering flavours), so a build system
rule was used to pass -DDYNAMIC_BY_DEFAULT to the C compiler so that
deriveConstants could infer it.
This commit fully removes support for dynamic-by-default. If someone wants to reimplement something like this, it would probably need to move the logic in the compiler.
(Doing this would probably need some refactoring of the way the compiler handles DynFlags: DynFlags are used to store and to pass enabled ways to many parts of the compiler. It can be set by command-line flags, GHC API, global settings. In multi-target GHC, we will use DynFlags to load the target platform and its constants: but at this point with the current DynFlags implementation we can't easily update the existing DynFlags with target-specific options such as dynamic-by-default without overriding ways previously set by the user.)