Common makefile (gasp) stuff

[Andrei Alexandrescu via Digitalmars-d]

Hey folks,

Over the time a number of stuff has become quite duplicated across our 
makefiles for dmd, druntime, and phobos.

These include fetching OS and model but (newer) general-purpose macros 
for e.g. rebasing repos, see 
https://github.com/D-Programming-Language/dlang.org/blob/master/posix.mak#L37.

I think it's time to reduce clutter and duplication by migrating such 
common stuff into one common.mak file. Question is, where should that 
file sit? One candidate is tools/ but it's not really a tool. Another 
candidate is dmd/src/ because that would be the first thing anyone 
depends on.

Ideas?


Andrei