Analyse following RPM build log snippet. Describe contents accurately, without speculation or suggestions for resolution.            
Your analysis must be as concise as possible, while keeping relevant information intact.
Snippet: 
../src/backlight/backlight.c: In function âread_brightnessâ:
../src/backlight/backlight.c:389:21: error: unused variable âsubsystemâ [-Werror=unused-variable]
  389 |         const char *subsystem, *value;
      |                     ^~~~~~~~~
Build failed during compilation of src/backlight/backlight.c

There is an unused variable `subsystem`