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.
Streamline the travis deploy builds; when TRAVIS_TAG is set
we'll run `--release` builds.
Don't error out in fontconfig/build.rs if fontconfig is not
installed. This makes it possible to `cargo test --all` again
on the mac at the cost of potentially making it harder to troubleshoot
problems with not having fontconfig installed on linux.
However: the get-deps script is responsible for installing that.
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.
