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/.travis.yml

@@ -0,0 +1,78 @@
+language: c
+compiler: clang
+branches:
+    only:
+        - ci
+        - master
+sudo: false
+dist: trusty
+addons:
+    apt:
+        packages:
+            - cmake
+            - libxrandr-dev
+            - libxinerama-dev
+            - libxcursor-dev
+            - libxi-dev
+matrix:
+    include:
+        - os: linux
+          env:
+              - BUILD_SHARED_LIBS=ON
+              - CFLAGS=-Werror
+        - os: linux
+          env:
+              - BUILD_SHARED_LIBS=OFF
+              - CFLAGS=-Werror
+        - os: linux
+          sudo: required
+          addons:
+              apt:
+                  packages:
+                      - libwayland-dev
+                      - libxkbcommon-dev
+                      - libegl1-mesa-dev
+          env:
+              - USE_WAYLAND=ON
+              - BUILD_SHARED_LIBS=ON
+              - CFLAGS=-Werror
+        - os: linux
+          sudo: required
+          addons:
+              apt:
+                  packages:
+                      - libwayland-dev
+                      - libxkbcommon-dev
+                      - libegl1-mesa-dev
+          env:
+              - USE_WAYLAND=ON
+              - BUILD_SHARED_LIBS=OFF
+              - CFLAGS=-Werror
+        - os: osx
+          env:
+              - BUILD_SHARED_LIBS=ON
+              - CFLAGS=-Werror
+        - os: osx
+          env:
+              - BUILD_SHARED_LIBS=OFF
+              - CFLAGS=-Werror
+script:
+    - if grep -Inr '\s$' src include docs tests examples CMake *.md .gitattributes .gitignore; then echo Trailing whitespace found, aborting.; exit 1; fi
+    - mkdir build
+    - cd build
+    - if test -n "${USE_WAYLAND}";
+          then wget https://mirrors.kernel.org/ubuntu/pool/universe/e/extra-cmake-modules/extra-cmake-modules_5.38.0a-0ubuntu1_amd64.deb;
+          sudo dpkg -i extra-cmake-modules_5.38.0a-0ubuntu1_amd64.deb;
+          git clone git://anongit.freedesktop.org/wayland/wayland-protocols;
+          pushd wayland-protocols;
+          git checkout 1.12 && ./autogen.sh --prefix=/usr && make && sudo make install;
+          popd;
+      fi
+    - cmake -DCMAKE_VERBOSE_MAKEFILE=ON -DBUILD_SHARED_LIBS=${BUILD_SHARED_LIBS} -DGLFW_USE_WAYLAND=${USE_WAYLAND} ..
+    - cmake --build .
+notifications:
+    email:
+        recipients:
+            - ci@glfw.org
+    on_success: never
+    on_failure: always