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 6.1 Basics of Variable References
 To substitute a variable's value, write a dollar sign followed by the
 name of the variable in parentheses or braces: either `$(foo)' or
 `${foo}' is a valid reference to the variable `foo'.  This special
 significance of `$' is why you must write `$$' to have the effect of a
 single dollar sign in a file name or command.
    Variable references can be used in any context: targets,
 prerequisites, commands, most directives, and new variable values.
 Here is an example of a common case, where a variable holds the names
 of all the object files in a program:
      objects = program.o foo.o utils.o
      program : $(objects)
              cc -o program $(objects)
      $(objects) : defs.h
    Variable references work by strict textual substitution.  Thus, the
      foo = c
      prog.o : prog.$(foo)
              $(foo)$(foo) -$(foo) prog.$(foo)
 could be used to compile a C program `prog.c'.  Since spaces before the
 variable value are ignored in variable assignments, the value of `foo'
 is precisely `c'.  (Don't actually write your makefiles this way!)
    A dollar sign followed by a character other than a dollar sign,
 open-parenthesis or open-brace treats that single character as the
 variable name.  Thus, you could reference the variable `x' with `$x'.
 However, this practice is strongly discouraged, except in the case of
 the automatic variables ( Automatic Variables).
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