Change hardcoded path to tzdata directory

[kinke]

On Saturday, 22 December 2018 at 16:51:27 UTC, Thomas Mader wrote:
> Is LDC using this Makefile to build Phobos

Nope.

> is it possible to specify that option in the LDC build somehow 
> or do I need to implement a solution for LDC too?

You can add arbitrary LDC compile flags for druntime/Phobos via 
`-DD_FLAGS=...` in the CMake command line, no need to hack a 
Makefile.
Your PR was merged into master, not stable, which means chances 
are this isn't going to land before 2.085, so you'll have to 
adapt LDC's Phobos until then.