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:     exportThis tells `make' that variables which are not explicitly mentioned inan `export' or `unexport' directive should be exported.  Any variablegiven in an `unexport' directive will still _not_ be exported.  If youuse `export' by itself to export variables by default, variables whosenames contain characters other than alphanumerics and underscores willnot be exported unless specifically mentioned in an `export' directive.   The behavior elicited by an `export' directive by itself was thedefault in older versions of GNU `make'.  If your makefiles depend onthis 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.  Thiswill be ignored by old `make's, while the `export' directive will causea syntax error.     Likewise, you can use `unexport' by itself to tell `make' _not_ toexport variables by default.  Since this is the default behavior, youwould 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 recipesand not for others.  The last `export' or `unexport' directive thatappears by itself determines the behavior for the entire run of `make'.   As a special feature, the variable `MAKELEVEL' is changed when it ispassed down from level to level.  This variable's value is a stringwhich 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 asub-sub-`make', and so on.  The incrementation happens when `make' setsup the environment for a recipe.   The main use of `MAKELEVEL' is to test it in a conditional directive(*note Conditional Parts of Makefiles: Conditionals.); this way you canwrite a makefile that behaves one way if run recursively and anotherway 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 awhitespace-separated list of file names.  This variable, if defined inthe outer-level makefile, is passed down through the environment; thenit serves as a list of extra makefiles for the sub-`make' to readbefore the usual or specified ones.  *Note The Variable `MAKEFILES':MAKEFILES Variable.