This makefile will compile and create an executable file called GCD in the current directory by default. If you want to make this into a systemwide command, type sudo make install. Once done, the executable will be copied to /usr/local/bin directory.
Using this command requires an argument which can hold a unsigned long long int (128 bit) value.
To execute the command type with local executable type:
./GCD <number-1> <number-2>
where <number-*> is the number that you want to check.
If sudo make install has been used, it would run if you type:
GCD <number-1> <number-2>
This function uses Euclidean Algorithm to find out the greatest common divisor of 2 numbers.
The prime.h library also has some extra and very useful functions that you might like to try.
Note: The makefile uses GNU make format wildcards, therefore in some BSD systems it would potentially not compile properly. I am trying to figure out a way to make it more cross platform so that this works in most *nix systems.