Add test.cpp to test C++ API

Makefile

@@ -1,16 +1,20 @@
 prefix = /usr/local
 bindir = $(prefix)/bin
 CFLAGS = -std=c99 -Wall -Wextra -pedantic
+CXXFLAGS = -std=c++20 -Wall -Wextra -pedantic
 
 .DEFAULT_GOAL := test
 
 
 clean:
-	rm -f test
+	rm -f test testcpp
 
 test: test.c
 	$(CC) test.c -g $(CFLAGS) -o test
 
+testcpp: test.cpp
+	$(CXX) test.cpp -g $(CXXFLAGS) -o testcpp
+
 check: test
 	./test.sh