Compiler setup for exactpred.c

From @hugoledoux's [comment](https://github.com/georust/geo/issues/380#issuecomment-571143533):
Regarding incorrect results when input is as follows:

p_a: (x: -0.0000000000000000033732417326292235, y: -710.1201610753293)
p_b: (x: -0.0000000000000006803180046827075, y: 0.0000000000000007474653075557756)
p_c: (x: -5921927286181025000000000, y: -70060321899175880000000000000)
predicates.c orient2d: 0
exactpred.rs orient2d: 4205279958339305000000000000

> the answer from predicates.c is certainly completely wrong. It returns 0 and that would mean the 3 points are collinear, and they are clearly not.
The 3 points abc, in that order, have a CCW orientation, which should be positive, which is what exactpred.rs returns.
Either predicates.c is wrong (which I doubt), or you didn't setup correctly the constant for your compiler, see details here.