adding clang builds

Author: bernhardmgruber

.travis.yml

@@ -4,8 +4,15 @@ cache: ccache
 
 addons:
   apt:
-    update: true
+    sources:
+      - sourceline: "ppa:ubuntu-toolchain-r/test"
+      - sourceline: "deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-9 main"
+        key_url: "https://apt.llvm.org/llvm-snapshot.gpg.key"
     packages:
+      - clang-9
+      - clang-tidy-9
+      - clang-format-9
+      - g++-9
       - libboost-all-dev
       - libglfw3-dev
       - libglew-dev
@@ -14,6 +21,16 @@ addons:
       - xorg-dev # needed by glfw3
       - libglu1-mesa-dev # needed by glfw3
 
+compiler:
+  - gcc
+  - clang
+
+install:
+  - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-9 100
+  - sudo update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-9 100
+  - sudo update-alternatives --install /usr/bin/clang-tidy clang-tidy /usr/bin/clang-tidy-9 100
+  - sudo update-alternatives --install /usr/bin/clang-format clang-format /usr/bin/clang-format-9 100
+
 script:
   - mkdir build
   - cd build