Contains the current C compiler. Defaults to cc. 
    Special options which are added to the built-in C rule. (See next page.) 
    Full name of the current target. 
    A list of files for current dependency which are out-of-date. 
    The source file of the current (single) dependency. 

You can also manipulate the way these macros are evaluated, as follows, assuming that OBJS = data.o io.o main.o, using $(OBJS:.o=.c) within the Makefile substitutes .o at the end with .c, giving you the following result: data.c io.c main.c

By itself, make knows already that in order to create a .o file, it must use cc -c on the corresponding .c file.