travis: add Doxygen integration build job

Add doxygen build job to `travis.yml`.

Create doxygen build function:
1. Download stable Doxygen version (1.8.13).
2. Build doxygen.
3. Generate documentation.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>

.travis.yml

@@ -5,12 +5,19 @@ sudo: required
 os: linux
 
 env:
+  global:
+    secure: "e2aIzAsFnISWA+43i6tF/hS3l6k4eI/k7/5ZBzrpcPxT2R/PqVL1AcDndBQFLFVAX9lACnMBIEhK8jxvK2VGNwACp/gLG4RNyB5oE+HQqcUY1ntdjVmAAVzDKzG23mPlXIQzGFeZ861BuiO4GSqEBsd2c0Nm7jXf2rSuHizkId8="
   matrix:
     - BUILD_TYPE=astyle
     - BUILD_TYPE=cppcheck
     - BUILD_TYPE=ad9361_generic
     - BUILD_TYPE=ad9361_linux
     - BUILD_TYPE=drivers
+    - BUILD_TYPE=doxygen
+
+before_install:
+  - export DEPS_DIR="${TRAVIS_BUILD_DIR}/deps"
+  - mkdir -p ${DEPS_DIR}
 
 script:
   - ./ci/travis/run_build.sh

ci/travis/run_build.sh

@@ -29,4 +29,30 @@ build_drivers() {
     make -C ./drivers -f Makefile
 }
 
+build_doxygen() {
+    sudo apt-get install -y graphviz
+    # Install a recent version of doxygen
+	DOXYGEN_URL="https://sourceforge.net/projects/doxygen/files/rel-1.8.13/doxygen-1.8.13.src.tar.gz/"
+	cd ${DEPS_DIR}
+	[ -d "doxygen" ] || {
+		mkdir doxygen && wget --quiet -O - ${DOXYGEN_URL} | tar --strip-components=1 -xz -C doxygen
+	}
+
+    # Install Doxygen
+    cd doxygen
+    mkdir -p build && cd build
+    cmake ..
+    make -j${NUM_JOBS}
+    sudo make install
+    cd ../..
+
+    # Build Documentation
+    cd ${TRAVIS_BUILD_DIR}/doc
+    mkdir -p build && cd build
+    cmake ..
+    cd ../..
+
+    ./ci/travis/doxygen.sh
+}
+
 build_${BUILD_TYPE}