Add dependency control to posix.mak

Improvements:

* fix documentation at the top
* eliminate `MODEL_FLAG`
* eliminate the case whern `BUILD` is undefined, leading to major simplifications. Now `BUILD` defaults to release.
* eliminate`DDOC_FLAGS`
* eliminate `win32remote` (hasn't worked in a long time)
* defaults `DMD` to `../dmd/src/dmd` so as to pick the newest dmd, not the one used in production
* allow parallel builds of PIC and PDC libraries on linux
* add true dependency tracking of phobos modules to other phobos and druntime modules, so as to minimize rebuilds when unittesting
* delete targets of the form `$(ROOT)/unittest/%$(DOTEXE)` which were not actually useful anymore
* now `make unittest` runs unittests every time run (previously if would only run the first time after a build) - this is better because there may be unittests that fail intermittenly
* added convenience phony targets of the form `unittest/std/algorithm.d`, which will test the respective module(s)