Test with C99 and C++11

We need va_copy which is only in C++11 and newer.

Makefile

···
 CFLAGS=-Wall -Wextra -Wpedantic
+CXXFLAGS=$(CFLAGS)
 
-test: tests
-	./tests
+test: tests_c tests_cpp
+	./tests_c
+	./tests_cpp
 
-tests: fmt.h tests.o
-	$(CC) $(CFLAGS) tests.o -o tests
+tests_c: fmt.h tests.c
+	$(CC) -std=c99 $(CFLAGS) tests.c -o tests_c
+
+tests_cpp: fmt.h tests.c
+	$(CXX) -std=c++11 $(CXXFLAGS) tests.c -o tests_cpp
 
 format:
 	clang-format -i fmt.h tests.c
 
-.PHONY: test format
+clean:
+	rm tests_c tests_cpp
+
+.PHONY: test format clean

fmt.h

···
   }
   result.length = (size_t)size;
   // +1 to include NUL
-  result.str = malloc(size + 1);
+  result.str = (char *)malloc(size + 1);
   if (result.str == NULL) {
     return result;
   }