An argument that contains `=' specifies the value of a variable: `v=x' sets the value of the variable v to x. If you specify a value in this way, all ordinary assignments of the same variable in the makefile are ignored; we say they have been overridden by the command line argument.
The most common way to use this facility is to pass extra flags to
compilers. For example, in a properly written makefile, the variable
CFLAGS is included in each command that runs the C compiler, so a
file `foo.c' would be compiled something like this:
cc -c $(CFLAGS) foo.c
Thus, whatever value you set for CFLAGS affects each compilation
that occurs. The makefile probably specifies the usual value for
CFLAGS, like this:
CFLAGS=-g
Each time you run make, you can override this value if you
wish. For example, if you say `make CFLAGS='-g -O'', each C
compilation will be done with `cc -c -g -O'. (This illustrates
how you can use quoting in the shell to enclose spaces and other
special characters in the value of a variable when you override it.)
The variable CFLAGS is only one of many standard variables that
exist just so that you can change them this way. See section Variables Used by Implicit Rules, for a complete list.
You can also program the makefile to look at additional variables of your own, giving the user the ability to control other aspects of how the makefile works by changing the variables.
When you override a variable with a command argument, you can define either a recursively-expanded variable or a simply-expanded variable. The examples shown above make a recursively-expanded variable; to make a simply-expanded variable, write `:=' instead of `='. But, unless you want to include a variable reference or function call in the value that you specify, it makes no difference which kind of variable you create.
There is one way that the makefile can change a variable that you have
overridden. This is to use the override directive, which is a line
that looks like this: `override variable = value'
(see section The override Directive).