Link with what library ?

The -l compiler flag tells the compiler to search through a given library (either a shared (.so) or a ranlib library (.a) for otherwise undefined symbols. So for example if you wish to call sqrt you need to tell the C compiler to add the library with the object code for sqrt using -lm (N.B. g++ adds the math library automatically, so you don't have to, but gcc does not). So to link in functions defined in the bluetooth library, you need to add -lbluetooth to your compile command.

You will also need to add the libbluetooth-dev package to your system using apt install libbluetooth-dev

Your shell snippet above suggest you're working as root (shell prompt # ). This is generally frowned upon, and might be dangerous. Almost every experienced system administrator has at one time done something like 'rm -rf *' as root, with disastrous consequences. Use root only when you need to, and then you should get in the habit of using sudo to perform the system admin functions, like installing or updating packages.