Files:
demo_fn.h - define the functions to calculate value and derivative; 
	    also define the function parameters for the general quadratic equation, f(x) = a x^2 + b x + c
demo_fn.c - function definitions
brent.c - driver function for brent method

Compile & Link:
gcc -c demo_fn.c
gcc -c brent.c
gcc -o brent brent.o -lgsl -lgslcblas -lm

Run
./brent