*.mak: Change the meaning of DMD_DIR to the path of the compiler source

[the-horo]

Change the meaning of DMD_DIR from the path of the dmd repository, including the compiler and druntime, to the path of the compiler source. This becomes consistent with its usage in druntime's makefiles.

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[RazvanN7]

This PR seems to add complexity to the paths in the makefile (by adding an extra ..) rather than simplify anything.

[the-horo]

I ask for this change because of the layout of the linux source archives at https://downloads.dlang.org where the 3 main components of dmd (the compiler, druntime and phobos) are found under src/{dmd,druntime,phobos}. Being able to specify the directory of the compiler sources will help avoid having to mv directories in a way that include $(DMD_DIR)/compiler/src/osmodel.mak is satisfied.

Technically, this is not an issue because I already have to move the directories to satisfy include $(DMD_DIR)/src/osmodel.mak, the line prior to the druntime and dmd merge, (src/dmd would have to become dmd/src).

Merging this PR would simplify my workflow a little bit but, given that this has been an issue since the dmd and druntime merge, and I didn't find anyone else requesting such a change perhaps not that many people are affected, in which case I understand the complexity I add is not worth it. In this case I would still like to add a comment explaining how the DMD_DIR variable has a different meaning in phobos/posix.mak compared to druntime/posix.mak though it has the same name. What do you think?