Add GLFW source tree to src/external

We need the CMake stuff for wayland configuration.
Otherwise, we would have to replicate that ourselves.
This is the full 7ef34eb06de54dd9186d3d21a401b2ef819b59e7 tree except
for tests/ examples/ and docs/

src/external/glfw/.appveyor.yml

@@ -0,0 +1,23 @@
+branches:
+    only:
+        - ci
+        - master
+skip_tags: true
+environment:
+    CFLAGS: /WX
+    matrix:
+        - BUILD_SHARED_LIBS: ON
+        - BUILD_SHARED_LIBS: OFF
+matrix:
+    fast_finish: true
+build_script:
+    - mkdir build
+    - cd build
+    - cmake -DCMAKE_VERBOSE_MAKEFILE=ON -DBUILD_SHARED_LIBS=%BUILD_SHARED_LIBS% ..
+    - cmake --build .
+notifications:
+    - provider: Email
+      to:
+        - ci@glfw.org
+      on_build_failure: true
+      on_build_success: false