Use CXXFLAGS when building C++.

https://github.com/uniconproject/unicon/commit/adc981124123da5bc42b22a96fbe1c3c195031b5#diff-68897f4ce93462790f613bf47f5b2ef00c621769faec599530ff3e5c252a408a
--- src/common/Makefile.orig	2020-12-21 16:08:43
+++ src/common/Makefile	2026-03-27 11:24:34
@@ -89,7 +89,7 @@
 drawstring3d.$(O): drawstring3d.cc
 	@if test "`which $(CXX)`" != "" ; then \
 		echo "Got C++, building drawstring3d.o"; \
-		$(CXX) -c $(CPPFLAGS) $(CFLAGS) -I/usr/include/freetype2 -I/opt/X11/include/freetype2 -o drawstring3d.o drawstring3d.cc; \
+		$(CXX) -c $(CPPFLAGS) $(CXXFLAGS) -I/usr/include/freetype2 -I/opt/X11/include/freetype2 -o drawstring3d.o drawstring3d.cc; \
 	else \
 		echo "No C++, too bad"; \
 		echo "extern void syserr(char*); int cpp_drawstring3d(double x, double y, double z, char *s, char *f, int t, int size, void **tfont) { syserr(\"no C++; it is required for 3d fonts\"); return -1; } " > drawstring3d.c ; \