When silent rules are disabled, don't hide the command to build drawstring3d.o.

https://github.com/uniconproject/unicon/issues/575
--- src/common/Makefile.orig	2026-04-03 06:56:39.000000000 -0500
+++ src/common/Makefile	2026-04-03 07:00:21.000000000 -0500
@@ -89,3 +89,3 @@
 drawstring3d.$(O): drawstring3d.cc
-	@if test "`which $(CXX)`" != "" ; then \
+	$(SLNT)if test "`which $(CXX)`" != "" ; then \
 		echo "Got C++, building drawstring3d.o"; \