Add a CI pass for pathfinder_c on Linux

This crate has often been broken by macOS changes, this patch avoids it being
broken again in the future.

See 42289ee or
9998fe4 for examples of such breakage.

Author: linkmauve

.travis.yml

@@ -9,6 +9,7 @@ addons:
     - libegl1-mesa-dev
     - libgtk-3-dev
     - libsdl2-dev
+    - libglfw3-dev
     - cmake
 script:
   - rustup target add aarch64-unknown-linux-gnu
@@ -19,6 +20,10 @@ script:
   - cd ..
   - cargo build
   - cargo test
+  - cd c
+  - cargo cinstall --prefix=$TRAVIS_HOME/.fake-install
+  - cd ../examples/c_canvas_glfw_minimal
+  - PKG_CONFIG_PATH=$TRAVIS_HOME/.fake-install/lib/pkgconfig make
 env:
   global:
     - HARFBUZZ_SYS_NO_PKG_CONFIG=true