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(make.info)Overriding

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Overriding Variables
====================

   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.  Note: 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' (Note: The
`override' Directive.).