Fix object compilation target in Makefile

The target on line 60 of the Makefile isn't currently being used. Running `make` falls back to the built-in rules and running `make -r` errors out.

This patch fixes the dep paths and also makes it impossible to omit `-c` from `CFLAGS`, as the built-ins [do](http://www.gnu.org/software/make/manual/html_node/Catalogue-of-Rules.html).