Fix hang on startup on some Linux systems

When the fontconfig dev package is not installed, servo-fontconfig will fall back to building
its own copy, which may be incompatible with what's on the system. The
result is a horrible start-up time and this patch opts to instead require fontconfig
to be available at build time and error out otherwise.

Fixes issue #88

sixtyfps_runtime/corelib/Cargo.toml

@@ -51,5 +51,10 @@ wasm-bindgen = { version = "0.2" }
 font-kit = "0.10"
 pathfinder_geometry = "0.5.1"
 
+# Require font-config from the system. Issue #88 indicates that the copy provided by servo-fontconfig may be incompatible
+# with distros at times.
+[target.'cfg(not(any(target_family = "windows", target_os = "macos", target_os = "ios", target_arch = "wasm32")))'.dependencies]
+servo-fontconfig-sys = { version = "5.1", features = [ "force_system_lib" ] }
+
 [dev-dependencies]
 pin-weak = "1"