Conditional compilation in C

How exactly does conditional compilation works in C?

I'm using GMP to do some calculations in a C program;
but if GMP isn't available in the machine, I could do the calculations with some handmade functions.

Any help would be great. Thanks in advance!

>>13
Haven't you ever read the source code to a major piece of software? If you have, you wouldn't have made such an ignorant and idiotic statements.