MAKEFILES
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 (see section Including Other Makefiles). In addition, the
default goal is never taken from one of these makefiles 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 (see section Recursive Use of make). 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 (see section Searching Directories for Dependencies).
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. See section Including Other Makefiles.