Sometimes “Nothing to be done for all” error can be caused by spaces before command in makefile rule instead of tab. Please ensure that you use tabs instead of spaces inside of your rules.
all:
instead of
all:
$(CC) $(CFLAGS) …
Please see the GNU make manual for the rule syntax description: https://www.gnu.org/software/make/manual/make.html#Rule-Syntax
Remove the hello file from your folder and try again.
The all target depends on the hello target. The hello target first tries to find the corresponding file in the filesystem. If it finds it and it is up to date with the dependent files—there is nothing to do.