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!

>>27
I'm not the foolholio that is making retarded ass statements. Why don't you just give google a rest, and like, try and write some actual C code.