COMP: Display a warning if doxygen is not found

A warning was already displayed when DOT wasn't found.

Documentation/CMakeLists.txt

@@ -59,8 +59,8 @@ IF( DOXYGEN_FOUND )
       )
     
   ELSE( DOXYGEN_DOT_FOUND )
-    MESSAGE( WARNING
-      "Warning: Dot not found - Documentation will not be created"
-      )
+    MESSAGE( WARNING "Warning: Dot not found - Documentation will not be created")
   ENDIF( DOXYGEN_DOT_FOUND )
+ELSE( DOXYGEN_FOUND )
+  MESSAGE( WARNING "Warning: Doxygen not found - Documentation will not be created")
 ENDIF( DOXYGEN_FOUND )