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 (*note 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 line of the recipe.  The
sub-`make', in turn, uses the environment to initialize its table of
variable values.  *Note 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.  *Note
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.
*Note Options/Recursion::.

   Variables are _not_ normally passed down if they were created by
default by `make' (*note 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

and

     export VARIABLE := value

has the same result as:

     VARIABLE := value
     export VARIABLE

   Likewise,

     export VARIABLE += value

is just like:

     VARIABLE += value
     export VARIABLE

*Note Appending More Text to Variables: Appending.