To build the parallel version on Mac just:
gcc password-crack-parallel.c -o bin/password-crack-parallel -framework OpenCL
On Linux (Ubuntu):
sudo apt install ocl-icd-opencl-dev
gcc password-crack-parallel.c -o bin/password-crack-parallel -lOpenCL
To build the serial version on Mac or Linux just:
gcc password-crack.c -o bin/password-crack
Just run the generated binaries from within the bin directory and enter the password to be "cracked" when requested.
Parallel GPU version will start making a difference with a password with size 5.