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!

If you do it during the compilation stage, you will not be able to distribute your binary. It is better to handle it at runtime, either via weak linking on platforms that support it, or by manually loading the dynamic library.

Though realistically, if you need GMP your own replacements aren't going to be worth shit.