Target oldest supported OS by default

[Jacob Carlborg]

I just had an issue with a binary compiled on Mac OS X 10.8 not running 
on any older version. When I found the problem it was an easy fix adding 
the following linker flags:

-L-macosx_version_min -L10.6
Would it be a good idea if DMD targeted the oldest supported OS by 
default, that is, when invoking GCC to link?

--
/Jacob Carlborg