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3.4 The Variable 'MAKEFILES'
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If the environment variable 'MAKEFILES' is defined, 'make' considers its
value as a list of names (separated by whitespace) of additional
makefiles to be read before the others.  This works much like the
'include' directive: various directories are searched for those files
(*note Including Other Makefiles: Include.).  In addition, the default
goal is never taken from one of these makefiles (or any makefile
included by them) and it is not an error if the files listed in
'MAKEFILES' are not found.

   The main use of 'MAKEFILES' is in communication between recursive
invocations of 'make' (*note Recursive Use of 'make': Recursion.).  It
usually is not desirable to set the environment variable before a
top-level invocation of 'make', because it is usually better not to mess
with a makefile from outside.  However, if you are running 'make'
without a specific makefile, a makefile in 'MAKEFILES' can do useful
things to help the built-in implicit rules work better, such as defining
search paths (*note Directory Search::).

   Some users are tempted to set 'MAKEFILES' in the environment
automatically on login, and program makefiles to expect this to be done.
This is a very bad idea, because such makefiles will fail to work if run
by anyone else.  It is much better to write explicit 'include'
directives in the makefiles.  *Note Including Other Makefiles: Include.