Merge branch 'fix/doxygen-macro-for-cmake-greater-3.1' into 'master'

[doxygen] Add target only if doxygen was found

The dune macro used to add the target only adds it if doxygen was found.
Then the target was not know in add_custom_command which is an error
in CMake > v3.2.

See merge request !40

cmake/modules/DumuxDoxygen.cmake

@@ -2,8 +2,10 @@
 #
 # make sure, that the doxygen links to todo list, bibliography, etc. are correct
 MACRO (add_dumux_doxygen_target)
-  add_doxygen_target()
-  add_custom_command(TARGET doxygen_${ProjectName}
-                     POST_BUILD
-                     COMMAND ${CMAKE_SOURCE_DIR}/doc/doxygen/sanitizelinks.sh)
+  if(DOXYGEN_FOUND)
+    add_doxygen_target()
+    add_custom_command(TARGET doxygen_${ProjectName}
+                       POST_BUILD
+                       COMMAND ${CMAKE_SOURCE_DIR}/doc/doxygen/sanitizelinks.sh)
+  endif()
 ENDMACRO (add_dumux_doxygen_target)