Improve Travis: test on Linux and macOS

Keep the Doxygen run as before, but also test building and installing
using cmake on Linux and macOS with both gcc and clang.

.travis.yml

@@ -1,11 +1,22 @@
+language: cpp
 sudo: false
+compiler:
+  - gcc
+  - clang
+os:
+  - linux
+  - osx
 dist: xenial
+osx_image: xcode7.3
 
-branches:
-  only:
-    - master
-
-# Install dependencies
+matrix:
+  exclude:
+    - compiler: clang
+    - os: osx
+  include:
+    - env: BUILD_DOXYGEN="yes"
+      compiler: gcc
+      os: linux
       addons:
         apt:
           packages:
@@ -14,8 +25,6 @@ addons:
             - doxygen-latex
             - doxygen-gui
             - graphviz
- 
-    
       install:
         - pwd
         - doxygen --version
@@ -32,7 +41,6 @@ install:
         - travis_retry wget -O - ${DOXYGEN_URL} | tar --strip-components=1 -xz -C doxygen
         - export PATH=${DEPS_DIR}/doxygen/bin:${PATH}
         - doxygen --version
-
       script:
         - pwd
         - cd ${TRAVIS_BUILD_DIR}
@@ -41,6 +49,18 @@ script:
         - doxygen Doxyfile
         - echo "" > html/.nojekyll
 
+install:
+  - cmake --version
+
+script:
+  - mkdir build
+  - cd build
+  - cmake -DCMAKE_INSTALL_PREFIX=$HOME/ext ..
+  - cmake --build . --target install
+
+notifications:
+  email: false
+
 deploy:
   provider: pages
   verbose: true
@@ -50,3 +70,4 @@ deploy:
   github_token: $GH_REPO_TOKEN
   on:
     branch: master
+    condition: $BUILD_DOXYGEN = yes