propagate provided CXXFLAGS

Index: configure
--- configure.orig
+++ configure
@@ -3770,7 +3770,7 @@ $as_echo "#define HAVE_GCC_VISIBILITY_EXTS 1" >>confde
 
 	VISIBILITY_FLAGS="-fvisibility=hidden"
 	CFLAGS="$VISIBILITY_FLAGS $CFLAGS"
-	CXXFLAGS="$VISIBILITY_FLAGS -fvisibility-inlines-hidden $CFLAGS"
+	CXXFLAGS="$VISIBILITY_FLAGS -fvisibility-inlines-hidden $CXXFLAGS"
 fi
 
 ## Pass -DIN_LIBOFX to the compiler so we can detect it and include config.h
