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!

>>9
If you do it during the compilation stage, you will not be able to distribute your binary.

If >>2 would have gotten the conditional guards right, it would have been possible to do this during the compilation state, and like uh... thus be able to distrube the binary. Geeze. Both you and >>2 speak from inexperience or some shit like that.