Hi, I'm receiving the following error on darwin whenever I try to build a nix derivation from nixos/nixpkgs.
```
nix-build -KA croc
...
building '/nix/store/6czsxshkicfpcpyfwhlwqf542cz159ca-xcbuild-0.1.2-pre.drv'...
...
FAILED: Libraries/libcar/CMakeFiles/car.dir/Sources/Rendition.cpp.o
/nix/store/wmb4c95cyqryqs6adv9hw8gdbnbcffgj-clang-wrapper-7.1.0/bin/clang++  -Dcar_EXPORTS -I../Libraries/libcar/Headers -I../Libraries/ext/Headers -I../Libraries/libbom/Headers -I../Libraries/libutil/H
eaders -std=c++11 -fno-rtti -fno-exceptions -O3 -DNDEBUG -arch x86_64 -fPIC   -Wall -Werror -fcolor-diagnostics -MD -MT Libraries/libcar/CMakeFiles/car.dir/Sources/Rendition.cpp.o -MF Libraries/libcar/C
MakeFiles/car.dir/Sources/Rendition.cpp.o.d -o Libraries/libcar/CMakeFiles/car.dir/Sources/Rendition.cpp.o -c ../Libraries/libcar/Sources/Rendition.cpp
../Libraries/libcar/Sources/Rendition.cpp:29:10: fatal error: 'compression.h' file not found
#include <compression.h>
         ^~~~~~~~~~~~~~~
1 error generated.
```
Does anyone know how to solve this issue. Theres an [old issue](https://github.com/facebook/xcbuild/issues/48) filed against xcbuild that looks like this error but it should be resolved