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 5.7.2 Communicating Variables to a Sub-`make'
 Variable values of the top-level `make' can be passed to the sub-`make'
 through the environment by explicit request.  These variables are
 defined in the sub-`make' as defaults, but do not override what is
 specified in the makefile used by the sub-`make' makefile unless you
 use the `-e' switch ( Summary of Options Options Summary.).
    To pass down, or "export", a variable, `make' adds the variable and
 its value to the environment for running each command.  The sub-`make',
 in turn, uses the environment to initialize its table of variable
 values.   Variables from the Environment Environment.
    Except by explicit request, `make' exports a variable only if it is
 either defined in the environment initially or set on the command line,
 and if its name consists only of letters, numbers, and underscores.
 Some shells cannot cope with environment variable names consisting of
 characters other than letters, numbers, and underscores.
    The value of the `make' variable `SHELL' is not exported.  Instead,
 the value of the `SHELL' variable from the invoking environment is
 passed to the sub-`make'.  You can force `make' to export its value for
 `SHELL' by using the `export' directive, described below.  
 Choosing the Shell.
    The special variable `MAKEFLAGS' is always exported (unless you
 unexport it).  `MAKEFILES' is exported if you set it to anything.
    `make' automatically passes down variable values that were defined
 on the command line, by putting them in the `MAKEFLAGS' variable.
    Variables are _not_ normally passed down if they were created by
 default by `make' ( Variables Used by Implicit Rules Implicit
 Variables.).  The sub-`make' will define these for itself.
    If you want to export specific variables to a sub-`make', use the
 `export' directive, like this:
      export VARIABLE ...
 If you want to _prevent_ a variable from being exported, use the
 `unexport' directive, like this:
      unexport VARIABLE ...
 In both of these forms, the arguments to `export' and `unexport' are
 expanded, and so could be variables or functions which expand to a
 (list of) variable names to be (un)exported.
    As a convenience, you can define a variable and export it at the same
 time by doing:
      export VARIABLE = value
 has the same result as:
      VARIABLE = value
      export VARIABLE
      export VARIABLE := value
 has the same result as:
      VARIABLE := value
      export VARIABLE
      export VARIABLE += value
 is just like:
      VARIABLE += value
      export VARIABLE
  Appending More Text to Variables Appending.
    You may notice that the `export' and `unexport' directives work in
 `make' in the same way they work in the shell, `sh'.
    If you want all variables to be exported by default, you can use
 `export' by itself:
 This tells `make' that variables which are not explicitly mentioned in
 an `export' or `unexport' directive should be exported.  Any variable
 given in an `unexport' directive will still _not_ be exported.  If you
 use `export' by itself to export variables by default, variables whose
 names contain characters other than alphanumerics and underscores will
 not be exported unless specifically mentioned in an `export' directive.
    The behavior elicited by an `export' directive by itself was the
 default in older versions of GNU `make'.  If your makefiles depend on
 this behavior and you want to be compatible with old versions of
 `make', you can write a rule for the special target
 `.EXPORT_ALL_VARIABLES' instead of using the `export' directive.  This
 will be ignored by old `make's, while the `export' directive will cause
 a syntax error.  
    Likewise, you can use `unexport' by itself to tell `make' _not_ to
 export variables by default.  Since this is the default behavior, you
 would only need to do this if `export' had been used by itself earlier
 (in an included makefile, perhaps).  You *cannot* use `export' and
 `unexport' by themselves to have variables exported for some commands
 and not for others.  The last `export' or `unexport' directive that
 appears by itself determines the behavior for the entire run of `make'.
    As a special feature, the variable `MAKELEVEL' is changed when it is
 passed down from level to level.  This variable's value is a string
 which is the depth of the level as a decimal number.  The value is `0'
 for the top-level `make'; `1' for a sub-`make', `2' for a
 sub-sub-`make', and so on.  The incrementation happens when `make' sets
 up the environment for a command.
    The main use of `MAKELEVEL' is to test it in a conditional directive
 ( Conditional Parts of Makefiles Conditionals.); this way you can
 write a makefile that behaves one way if run recursively and another
 way if run directly by you.
    You can use the variable `MAKEFILES' to cause all sub-`make'
 commands to use additional makefiles.  The value of `MAKEFILES' is a
 whitespace-separated list of file names.  This variable, if defined in
 the outer-level makefile, is passed down through the environment; then
 it serves as a list of extra makefiles for the sub-`make' to read
 before the usual or specified ones.   The Variable `MAKEFILES'
 MAKEFILES Variable.
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