Don't accidentally create a subdirectory.

src/Makefile

@@ -51,7 +51,8 @@ lex.yy.cc: scanner.lex
 install:
 	cp libharplib.so $(PREFIX)/lib
 	cp harptool $(PREFIX)/bin
-	cp -r include $(PREFIX)/include/harp
+	mkdir -p $(PREFIX)/include/harp
+	cp include/* $(PREFIX)/include/harp
 
 clean:
 	rm -f *~ \#* *.o *.a *.so include/*~ include/\#* harptool