OS version symbols

[Johannes Pfau]

I started implementing the CPU version symbols:
https://gist.github.com/4701086

It doesn't mirror the C++ macro structure as we usually don't have to
overwrite the macro definitions for specific architectures. So the
generic definitions are in the top level OS files which are used for
every architecture. It's still possible to add architecture and OS
specific versions, see e.g. Android or GDC_MinGW64.


As a next step I'd like to remove target-ver-syms.sh, but there's one
problem: What to do about the TARGET_OSX,... preprocessor definitions?
Should I change all of those to runtime variables? Also: Should extra
code be added to the TARGET_OS_D_BUILTINS macros or could we just use
findCondition to check if version(OSX) is set and then use that result?