Use the requested C++ compiler and flags

https://github.com/uniconproject/unicon/pull/576
--- Makedefs.in.orig	2026-04-03 07:56:28.000000000 -0500
+++ Makedefs.in	2026-04-03 07:57:10.000000000 -0500
@@ -13,10 +13,12 @@
 # make sure to also change the optimization to level 0 or 1
 
 CC = @CC@
+CXX = @CXX@
 
 RTDIR=$(TOPDIR)/rt
 
 CFLAGS = @CFLAGS@ -I../../rt/include
+CXXFLAGS = @CXXFLAGS@
 LDFLAGS= -L../../rt/lib @LDFLAGS@
 LIBS= @LIBS@
 CPPFLAGS = @CPPFLAGS@