replace phobos makefile and tools makefile with D

[timotheecour]

[reposted here from D.learn on Jonathan M Davis' request]

Can we replace phobos' and tools' Makefile with D with following
advantages:

* no more 3 distinct files (posix.mak,win32.mak,win64.mak),
instead use version() to maximize code reuse.

* less verbosity / bugs / forward compatibility issues eg when a
module is added/renamed: eg could take advantage of D's
capabilities to iterate over modules / files instead of
explicitly listing them, which is fragile.

The D file could actually use some modules in phobos (so long it
doesn't use libphobos.a), plus another custom D module if need be.

The other Makefiles (druntime and dmd) are of course trickier
since it'd require boosstrapping.

In my experience writing D based makefiles is very easy, short
and clean.

Thanks for your comments!