(make) Environment

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 Variables from the Environment
 ==============================
 
    Variables in `make' can come from the environment in which `make' is
 run.  Every environment variable that `make' sees when it starts up is
 transformed into a `make' variable with the same name and value.  But
 an explicit assignment in the makefile, or with a command argument,
 overrides the environment.  (If the `-e' flag is specified, then values
 from the environment override assignments in the makefile.  
 Summary of Options Options Summary.  But this is not recommended
 practice.)
 
    Thus, by setting the variable `CFLAGS' in your environment, you can
 cause all C compilations in most makefiles to use the compiler switches
 you prefer.  This is safe for variables with standard or conventional
 meanings because you know that no makefile will use them for other
 things.  (But this is not totally reliable; some makefiles set `CFLAGS'
 explicitly and therefore are not affected by the value in the
 environment.)
 
    When `make' is invoked recursively, variables defined in the outer
 invocation can be passed to inner invocations through the environment
 ( Recursive Use of `make' Recursion.).  By default, only
 variables that came from the environment or the command line are passed
 to recursive invocations.  You can use the `export' directive to pass
 other variables.   Communicating Variables to a Sub-`make'
 Variables/Recursion, for full details.
 
    Other use of variables from the environment is not recommended.  It
 is not wise for makefiles to depend for their functioning on
 environment variables set up outside their control, since this would
 cause different users to get different results from the same makefile.
 This is against the whole purpose of most makefiles.
 
    Such problems would be especially likely with the variable `SHELL',
 which is normally present in the environment to specify the user's
 choice of interactive shell.  It would be very undesirable for this
 choice to affect `make'.  So `make' ignores the environment value of
 `SHELL' (except on MS-DOS and MS-Windows, where `SHELL' is usually not
 set.   Special handling of SHELL on MS-DOS Execution.)
 

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