patched Makefile to honor predefined standard variables (II)

Sorry, I missed the issue from pull request #1. Actually, the way the linking command is written, it's not meant to call the linker directly (in http://www.lisha.ufsc.br/teaching/os/exercise/hello.html there's an example how ld is called directly). Therefore I would recommend to also call CC instead of LD in this case. Setting LD to $(CC) would also do but I think it would be misleading as we want to call the compiler not the linker. Thank you!