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!

>>21
Responding to a substandard troll, but..
With only compile-time checking, a binary build with GMP support will not run on a machine without GMP installed (unless you're linking statically, which you shouldn't). With runtime checks, you can gracefully fall back with reduced functionality.