Forgetting to update the submodules is a commonly reported build
issue with an obscure error message:
```
CMake Error: The source directory "/home/USER/wezterm/target/debug/build/freetype-430fe24cb64c561c/out/zlib-src/zlib" does not appear to contain CMakeLists.txt.
```
this teaches the build.rs machinery to run the submodule update
if it looks like it is needed.
This is primarily for macos where the default freetype
installation is unable to render color emoji, but should also
help make things more consistent across the various platforms.
It's a little bit awkward on linux because the font-loader crate
pulls in the now-conflicting servo-font* crates. I've disabled
font-loader on linux systems; it's just calling fontconfig under
the covers anyway.
