Test multiple C++ standards; move tests to top level

Makefile

@@ -1,7 +1,23 @@
-.PHONY: test
-test:
-	$(MAKE) -C test
+# This Makefile is entirely for running unit tests for rcp.h
+
+INCLUDE := include
+STANDARDS := 11 14 17 20 23
+BINARIES := $(addprefix build/tests-, $(STANDARDS))
+
+.PHONY: all
+all: $(BINARIES)
+	@for std in $(STANDARDS); do \
+		echo "=== Test C++$$std ==="; \
+		build/tests-$$std; \
+		valgrind --leak-check=full --error-exitcode=1 \
+			--log-file=build/valgrind-$$std.log build/tests-$$std \
+			|| { echo "valgrind failed for C++$$std (see build/valgrind-$$std.log)"; exit 1; }; \
+	done
+
+build/tests-%: tests.cpp $(INCLUDE)/rcp.h
+	@mkdir -p $(dir $@)
+	$(CXX) -std=c++$* -g -Wall -I$(INCLUDE) -o $@ $<
 
 .PHONY: clean
 clean:
-	$(MAKE) -C test clean
+	rm -rf build