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 unittests/std/algorithm.d, which will test the respective module(s)