need a good way to determine the OS from a BASH script

[bob 295]

I'm in the process of debugging a Mac version of my SIMPL open source toolkit.   
With the GNU tools installed on the Mac the SIMPL toolkit compiles straight 
up without any conditional compilation ifdefs.    However, I've encountered 
an issue with the shmctl() function.    Linux allows a process to shmat() 
even if the segment was previously marked for destruction.   The Mac does 
not.   The reason for marking immediately after creation is to prevent shared 
memory leaks in cases of aborted processes.    As a result of this difference 
I'm going to have to introduce a conditional compile ifdef for the MACOSX.

Ideally I'd like to be able to detect that I'm about to compile on the MAC 
inside the Makefile (or at least in the install script).    Any ideas on how 
one could reliably detect a Linux OS as opposed to a Mac OS?

Thanks in advance for your help.

bob
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