replace phobos makefile and tools makefile with D

[timotheecour]

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.

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.