Build fails on Arch Linux due to Boost related issue

I'm trying to build Watchman (latest weekly release) on Arch Linux and I get lots of those errors:

```
CMake Error at CMakeLists.txt:525 (add_library):
Target "string" links to target "Boost::system" but the target was not
found. Perhaps a find_package() call is missing for an IMPORTED target, or
an ALIAS target is missing?
```

CMake complains about not being able to find Boost. I used the following build command:

```
python3 build/fbcode_builder/getdeps.py --allow-system-packages build --src-dir=. watchman --project-install-prefix watchman:/usr --num-jobs=12
```

Removing `--allow-system-packages` does not fix the problem (results in the same errors).