declare an automake conditional for each found module

@Christian: Could you please take a look at this?

[[Imported from SVN: r5602]]

m4/dune.m4

@@ -241,6 +241,8 @@ AC_DEFUN([DUNE_CHECK_MODULES],[
     with_[]_dune_module="no"
   fi
 
+  AM_CONDITIONAL(HAVE_[]_DUNE_MODULE, test x$HAVE_[]_DUNE_MODULE = x1)
+
   # reset previous flags
   CPPFLAGS="$ac_save_CPPFLAGS"
   LIBS="$ac_save_LIBS"