Makefiles

Makefiles allow you to automate the process of compiling and linking large, multi-file programs. By storing dependency information (e.g., complex.h is used by complex.c and ctest.c) and keeping track of which files have changed, Makefiles can quickly and painlessly execute only the minimal number of needed compiles.
make is the program that interprets makefiles. By default, make interprets the makefile called Makefile, but this can be overridden with the -f option.
The fundamental entry of a makefile is the dependency rule, which looks like this:
```
ctest : ctest.o complex.o
	gcc ctest.o complex.o -g -o ctest
```
This dependency rule tells make that ctest (the target) depends on the two files ctext.o and complex.o. If ctest doesn't exist, or if it is older than either ctest.o or complex.o, then it can be brought up to date by executing the gcc command.

A complete makefile for the complex number tester package might look like this:

ctest : ctest.o complex.o
        gcc ctest.o complex.o -g -o ctest
ctest.o : ctest.c complex.h
	gcc ctest.c -c -g
complex.o : complex.c complex.h
	gcc complex.c -c -g

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