The built-in implicit rules use several variables in their recipes so that, by changing the values of the variables, you can change the way the implicit rule works. For example, the variable CFLAGS controls the flags given to the C compiler by the implicit rule for C compilation.
example:
following CFLAGS will be used when implicit rule for CC is used.
CFLAGS = -O2 -Wall
refer:
https://www.gnu.org/software/make/manual/html_node/Implicit-Rules.html