( Setting

Info Catalog ( Values ( Using Variables ( Appending
 6.5 Setting Variables
 To set a variable from the makefile, write a line starting with the
 variable name followed by `=' or `:='.  Whatever follows the `=' or
 `:=' on the line becomes the value.  For example,
      objects = main.o foo.o bar.o utils.o
 defines a variable named `objects'.  Whitespace around the variable
 name and immediately after the `=' is ignored.
    Variables defined with `=' are "recursively expanded" variables.
 Variables defined with `:=' are "simply expanded" variables; these
 definitions can contain variable references which will be expanded
 before the definition is made.   The Two Flavors of Variables
    The variable name may contain function and variable references, which
 are expanded when the line is read to find the actual variable name to
    There is no limit on the length of the value of a variable except the
 amount of swapping space on the computer.  When a variable definition is
 long, it is a good idea to break it into several lines by inserting
 backslash-newline at convenient places in the definition.  This will not
 affect the functioning of `make', but it will make the makefile easier
 to read.
    Most variable names are considered to have the empty string as a
 value if you have never set them.  Several variables have built-in
 initial values that are not empty, but you can set them in the usual
 ways ( Variables Used by Implicit Rules Implicit Variables.).
 Several special variables are set automatically to a new value for each
 rule; these are called the "automatic" variables ( Automatic
    If you'd like a variable to be set to a value only if it's not
 already set, then you can use the shorthand operator `?=' instead of
 `='.  These two settings of the variable `FOO' are identical ( The
 `origin' Function Origin Function.):
      FOO ?= bar
      ifeq ($(origin FOO), undefined)
      FOO = bar
Info Catalog ( Values ( Using Variables ( Appending
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