In /usr/bin/sdl-config line 6:
libdir=${prefix}/lib/x86_64-linux-gnu
^----^ SC2034: libdir appears unused. Verify use (or export if used externally).


In /usr/bin/sdl-config line 20:
  -*=*) optarg=`echo "$1" | LC_ALL="C" sed 's/[-_a-zA-Z0-9]*=//'` ;;
               ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
  -*=*) optarg=$(echo "$1" | LC_ALL="C" sed 's/[-_a-zA-Z0-9]*=//') ;;


In /usr/bin/sdl-config line 32:
      echo $prefix
           ^-----^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
      echo "$prefix"


In /usr/bin/sdl-config line 39:
      echo $exec_prefix
           ^----------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
      echo "$exec_prefix"


In /usr/bin/sdl-config line 45:
      echo -I${prefix}/include/SDL -D_GNU_SOURCE=1 -D_REENTRANT
             ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
      echo -I"${prefix}"/include/SDL -D_GNU_SOURCE=1 -D_REENTRANT


In /usr/bin/sdl-config line 48:
      echo -L${prefix}/lib/x86_64-linux-gnu  -lSDL
             ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
      echo -L"${prefix}"/lib/x86_64-linux-gnu  -lSDL


In /usr/bin/sdl-config line 52:
      echo -L${prefix}/lib/x86_64-linux-gnu  -lSDL  -lpthread  -lm -ldl  -lasound -lm -ldl -lpthread -lpulse-simple -lpulse  -lX11 -lXext -L/usr/lib/x86_64-linux-gnu -lcaca -lpthread
             ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
      echo -L"${prefix}"/lib/x86_64-linux-gnu  -lSDL  -lpthread  -lm -ldl  -lasound -lm -ldl -lpthread -lpulse-simple -lpulse  -lX11 -lXext -L/usr/lib/x86_64-linux-gnu -lcaca -lpthread

For more information:
  https://www.shellcheck.net/wiki/SC2034 -- libdir appears unused. Verify use...
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...
  https://www.shellcheck.net/wiki/SC2006 -- Use $(...) notation instead of le...