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.
Merge pull request #117 from WebDrake/makefile-dflags

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