( Using Variables

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 6 How to Use Variables
 A "variable" is a name defined in a makefile to represent a string of
 text, called the variable's "value".  These values are substituted by
 explicit request into targets, prerequisites, commands, and other parts
 of the makefile.  (In some other versions of `make', variables are
 called "macros".)  
    Variables and functions in all parts of a makefile are expanded when
 read, except for the shell commands in rules, the right-hand sides of
 variable definitions using `=', and the bodies of variable definitions
 using the `define' directive.
    Variables can represent lists of file names, options to pass to
 compilers, programs to run, directories to look in for source files,
 directories to write output in, or anything else you can imagine.
    A variable name may be any sequence of characters not containing `:',
 `#', `=', or leading or trailing whitespace.  However, variable names
 containing characters other than letters, numbers, and underscores
 should be avoided, as they may be given special meanings in the future,
 and with some shells they cannot be passed through the environment to a
 sub-`make' ( Communicating Variables to a Sub-`make'
    Variable names are case-sensitive.  The names `foo', `FOO', and
 `Foo' all refer to different variables.
    It is traditional to use upper case letters in variable names, but we
 recommend using lower case letters for variable names that serve
 internal purposes in the makefile, and reserving upper case for
 parameters that control implicit rules or for parameters that the user
 should override with command options ( Overriding Variables
    A few variables have names that are a single punctuation character or
 just a few characters.  These are the "automatic variables", and they
 have particular specialized uses.   Automatic Variables.


* Reference                   How to use the value of a variable.
* Flavors                     Variables come in two flavors.
* Advanced                    Advanced features for referencing a variable.
* Values                      All the ways variables get their values.
* Setting                     How to set a variable in the makefile.
* Appending                   How to append more text to the old value
                                   of a variable.
* Override Directive          How to set a variable in the makefile even if
                                   the user has set it with a command argument.
* Defining                    An alternate way to set a variable
                                   to a verbatim string.
* Environment                 Variable values can come from the environment.
* Target-specific             Variable values can be defined on a per-target
* Pattern-specific            Target-specific variable values can be applied
                                   to a group of targets that match a pattern.
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