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!

>>12
Actually kodak-kun, i'm a project manager that tells you codemonkeys to make my projects for me. If anyone is the toilet scubbers then it's you.


Enjoying that enterprise java codemonkey job of yours? yeah go fuck off you piece of shit. You're not special, you're replaceable.