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!

Google for C preprocessor.

example:

#ifdef CAN_HAS_GMP
#define DO_SJIT gmp_do_shit
#else
#define DO_SJIT handmade_do_shit
#endif

void fun()
{
    DO_SJIT(some parameter shit);
}