Fix Issue 12205: update posix.mak to use local, recently-built druntime and phobos

This fixes https://d.puremagic.com/issues/show_bug.cgi?id=12205 by adding library/include paths to the locations where druntime and phobos have been built, plus a flag to prevent/suppress alignment errors in one of the builds.

In the event that the user wishes to build using the system `dmd` instead of the locally-built `../dmd/src/dmd`, this is trivial to achieve by overriding with `DMD=dmd`.  DFLAGS will also then be overridden with those from the system `dmd.conf`.