Naive question about system dependencies

[hholm@clark.net (Howard Holm)]

I have some experience using Ada in a classroom setting, but unfortunately 
little experience using it in any real-world settings.  I was considering how 
I would go about writing a program that was intended to be used in several 
different operating system environments (e.g. OS/2 and UNIX).  If I were using 
C, I would either use compiler generated macros, if they existed, or create 
some macros in the makefile that could be used to determine which system was 
the target (i.e. #ifdef OS2).  The most likely equivalent seemed to me to be 
the System_Name constant in the System package.  However, when I checked, the 
Gnat compiler simply uses "GNAT" for that definition.  So, my question is how 
do you go about accounting for differences in GUI's and the like?  Do you have 
a build directory that you copy implementation specific low-level packages 
into and out of for each build, or do you define a search path and have a 
collection of implementation specific directories and an independent 
directory? Or, did I overlook some other language featue to address this 
question?

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Howard Holm
hholm@clark.net