[duneproject] use ${} for correct variable expansion

Otherwise UNAME_HH was seen by the shell as the name of the variable,
but actually, the variable is called UNAME and _HH should be appended
after expansion of the variable.

bin/duneproject

@@ -781,12 +781,12 @@ EOF
 ################## dune/$NAME/$NAME.hh #################
 echo "- $PROJECT/dune/$NAME/$NAME.hh"
 cat> $PROJECT/dune/$NAME/$NAME.hh <<EOF
-#ifndef $UNAME_HH
-#define $UNAME_HH
+#ifndef ${UNAME}_HH
+#define ${UNAME}_HH
 
 // add your classes here
 
-#endif // $UNAME_HH
+#endif // ${UNAME}_HH
 EOF