GNU make manual 翻译( 一百六十五)

6.7 The `override' Directive
============================

If a variable has been set with a command argument (*note Overriding
Variables: Overriding.), then ordinary assignments in the makefile are
ignored.  If you want to set the variable in the makefile even though
it was set with a command argument, you can use an `override'
directive, which is a line that looks like this:

     override VARIABLE = VALUE

or

     override VARIABLE := VALUE

   To append more text to a variable defined on the command line, use:

     override VARIABLE += MORE TEXT

*Note Appending More Text to Variables: Appending.

   Variable assignments marked with the `override' flag have a higher
priority than all other assignments, except another `override'.
Subsequent assignments or appends to this variable which are not marked
`override' will be ignored.

   The `override' directive was not invented for escalation in the war
between makefiles and command arguments.  It was invented so you can
alter and add to values that the user specifies with command arguments.

   For example, suppose you always want the `-g' switch when you run the
C compiler, but you would like to allow the user to specify the other
switches with a command argument just as usual.  You could use this
`override' directive:

     override CFLAGS += -g

   You can also use `override' directives with `define' directives.
This is done as you might expect:

     override define foo =
     bar
     endef

*Note Defining Multi-Line Variables: Multi-Line.