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 9.5 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 also 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.   Variables Used
 by Implicit Rules Implicit Variables, 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' ( The
 `override' Directive Override Directive.).
 

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