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!

>>7
What happens if I use those flags in a makefile and then port this to a different OS that doesn't support GMP? Exactly. Perhaps you should shut up and try it before opening your mouth again.