Try to build documentation on travis, too

.travis.yml

@@ -35,6 +35,7 @@ addons:
       - check
       - libsubunit-dev
       - ninja-build
+      - doxygen
 
 before_install:
   # Install cmake

CMakeLists.txt

@@ -28,4 +28,6 @@ include(CPack)
 # Generate docs before creating source package
 include(src/Filelists.cmake)
 add_custom_target(dist COMMAND ${CMAKE_MAKE_PROGRAM} package_source)
-add_dependencies(dist lwipdocs)
+if (TARGET lwipdocs)
+  add_dependencies(dist lwipdocs)
+endif()

travis.sh

@@ -25,7 +25,19 @@ cd build
 # Build CMake
 ERR=$?
 if [ $ERR != 0 ]; then
-       echo "cmake build failed"
+       echo "cmake GENERATE failed"
        exit 33
 fi
 /usr/local/bin/cmake --build . -- -j 4
+ERR=$?
+if [ $ERR != 0 ]; then
+       echo "cmake build failed"
+       exit 33
+fi
+
+/usr/local/bin/cmake --build . --target lwipdocs
+ERR=$?
+if [ $ERR != 0 ]; then
+       echo "lwIP documentation failed"
+       exit 33
+fi