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!

>>17
I haven't made any statements in this thread until >>16

I'm tired of seeing your bullshit of telling other codemonkeys that they are toilet scrubbers when all you are toilet scrubbing codemonkeyes.