I am trying to revise a make file procedure for building a piece of
software that runs on SGI's IRIX OS to run instead on Linux. I want
documentation for using the -DSGI and -DUNDERSCORE parameters. These
parameters appear as options in files ending with ".makerc" when
invoking the C compiler. A good answer will point me toward readily
accessible documentation for this issue that explains 1) the
definition of the parameters and how they are used and 2) explains how
the make system recognizes these parameters and what to do with them
when it encounters them.

I have used Google to search for information, but I haven't found what
I want. Some of the problems I've encountered are that "make" is an
extremely common term and that "DSGI" is also a term associated with
SAS graphics software, which is not what I want.

My search turned up lots of occurrences of -DSGI, but all the ones
I've seen are simply listings of existing make files that use the
parameter without any explanation of *why* it's used. I've seen
instances of using other similar parameters like -DHP, -DDEC, -DX86,
etc. I don't see how the various make systems know how to recognize
and use these parameters. I have a copy of the O'Reilly book on make
(not the most current edition), and I haven't found any references to
such parameters. I can't understand how so many make procedures can be
using this parameter when I can't find the documentation for it.

I have found many fewer references to the -DUNDERSCORE parameter. Like
-DSGI, most of them are simply occurrences in make files without any
explanation. I did find one reference at
<http://ck-sg.p.lodz.pl/csds/html/zinstall.html> that suggests this
parameter has to do with what I've heard referred to on Windows
systems as "name mangling." It's how the C routine names are decorated
during compilation so that the linker can resolve references to them
from Fortran routines.