BUG: mantis 1386 - don't add InDoxygenGroup test if otb_source_dir is undefined

CMake/OTBModuleDoxygen.cmake

@@ -61,7 +61,7 @@ macro( otb_module_doxygen _name )
       @ONLY
     )
   endif()
-  if(NOT ${_name}_THIRD_PARTY AND EXISTS ${${_name}_SOURCE_DIR}/include)
+  if(OTB_SOURCE_DIR AND NOT ${_name}_THIRD_PARTY AND EXISTS ${${_name}_SOURCE_DIR}/include)
     if(PYTHON_EXECUTABLE)
       otb_add_test(NAME ${_name}InDoxygenGroup COMMAND ${PYTHON_EXECUTABLE} "${OTB_SOURCE_DIR}/Utilities/Doxygen/mcdoc.py" check ${_name} ${${_name}_SOURCE_DIR}/include)
       otb_memcheck_ignore(${_name}InDoxygenGroup)