also rerun Doxygen when da "modules" file has changed, as used for example by...

also rerun Doxygen when da "modules" file has changed, as used for example by dune grid do seed the module hierachy

[[Imported from SVN: r5547]]

am/doxygen

@@ -59,7 +59,7 @@ uninstall-local: uninstall-doxygen-data
 endif
 
 # what files does the doxygen-generated stuff depend on (find-syntax)
-DOXYGEN_DEPENDON = -name \*.cc -o -name \*.hh -o -name \*.png -o -name Doxyfile
+DOXYGEN_DEPENDON = -name \*.cc -o -name \*.hh -o -name \*.png -o -name Doxyfile -o -name modules
 
 # check dependency ourself to be robust
 doxygen-tag: FORCE Doxyfile $(DOXYGENHEADER) $(DOXYGENFOOTER)