druntime homebrew: setup and compile a single project with its own stdlib

[Dmitry Ponyatov]

I want to play with reimplementing some tiny set of standard D 
library, mostly for language learning, and need some advice
- is it possible to use `dub` both for building custom druntime 
and test app?
- or I must write some batch or Makefile and call `dmd` compiler 
directly onto every file?
- what `dmd`/`dub` command line options forces it to drop 
system-wide druntime/phobos and use custom local .so / .a library 
in place of it?