GDC phobos source path

[John Colvin via D.gnu]

How does gdc know where to look for druntime/phobos source code? 
I successfully built gdc for the first time on OS X, but it's 
looking at completely the wrong source code.

I have gdc manually (i.e. make install) installed in /opt/gdc and 
ldc installed via homebrew in /usr/local, but when I try and 
compile anything with gdc I get

/usr/local/Cellar/ldc/0.15.2-beta1/include/d/core/stdc/config.d:28:3: error: static if conditional cannot be at global scope
    static if( (void*).sizeof > int.sizeof )
    ^

which makes very little sense to me. At the very least I would 
have expected it to see /usr/local/include/d or 
/usr/local/include/d2