FindDoxygen: add optional ALL argument

Modules/FindDoxygen.cmake

@@ -69,6 +69,7 @@ Functions
 
     doxygen_add_docs(targetName
         [filesOrDirs...]
+        [ALL]
         [WORKING_DIRECTORY dir]
         [COMMENT comment])
 
@@ -91,6 +92,8 @@ Functions
   the :command:`add_custom_target` command used to create the custom target
   internally.
 
+  If ALL is set, the target will be added to the default build target.
+
   The contents of the generated ``Doxyfile`` can be customized by setting CMake
   variables before calling ``doxygen_add_docs()``. Any variable with a name of
   the form ``DOXYGEN_<tag>`` will have its value substituted for the
@@ -788,7 +791,7 @@ function(doxygen_list_to_quoted_strings LIST_VARIABLE)
 endfunction()
 
 function(doxygen_add_docs targetName)
-    set(_options)
+    set(_options ALL)
     set(_one_value_args WORKING_DIRECTORY COMMENT)
     set(_multi_value_args)
     cmake_parse_arguments(_args
@@ -1089,8 +1092,13 @@ doxygen_add_docs() for target ${targetName}")
     set(_target_doxyfile "${CMAKE_CURRENT_BINARY_DIR}/Doxyfile.${targetName}")
     configure_file("${_doxyfile_template}" "${_target_doxyfile}")
 
+    unset(_all)
+    if(${_args_ALL})
+        set(_all ALL)
+    endif()
+
     # Add the target
-    add_custom_target( ${targetName} VERBATIM
+    add_custom_target( ${targetName} ${_all} VERBATIM
         COMMAND ${CMAKE_COMMAND} -E make_directory ${_original_doxygen_output_dir}
         COMMAND "${DOXYGEN_EXECUTABLE}" "${_target_doxyfile}"
         WORKING_DIRECTORY "${_args_WORKING_DIRECTORY}"