howto identify os

[Francesco Zappa Nardelli]

Hello.

I am a novice user of autoconf, and I am sorry if my question is
really silly.

I am looking for a way to tell configure that the makefile it
eventually generates should include the file "linux.mak" if we are
building the software on Linux, and "macosx.mak" if we are building on
Darwin.  

How can I reliably detect the operating system of the machine the
software is built on?  

Thank you in advance.

Regards,
-francesco

ps: please, cc me as I am not subscribed to the list.