|Not sure exactly what your question is, but "make clean" should purge all the generated objects. To be honest you may get better answers at The BlueZ website[^]; that's where the knowledge is likely to be.
This is the key:
g++ -v -o "BLUE2_LINUX_19381" ./src/BLUE.o -lbluetooth
A pre-linker is creating a new library object called
BLUE2_LINUX_19381, which you can see later being included in the main link process.