There's a language standard that compilers are pretty much required to support if they want any market share, but the language definition also allows for non-standard extensions. Every compiler out there will take advantage of this allowance, so code that uses extensions will not be portable across compilers. The same goes with system APIs and third party libraries. However, if you use the standard language, your code should compile everywhere and run as expected.

i wonder why anyone would want to write non-standard code.

Depending whom you work for, there might be a favorite compiler in the shop. Many schools teach on old and outdated compilers, often teachers are most reluctant to change and rewrite their material, the list goes on ...

I have found the Dev C++ IDE reasonably generic in the Windows and Linux world, conforming to most C++ standards with their GCC/G++ combination of compilers.

Not just that, but unless you want to limit yourself to a very small library of functions indeed you're going to have to use compiler specific libraries at some point.
The trick is to use those libraries in such a way that the largest possible part of your code does not directly depend on them.

Example are GUI libraries. Those are pretty much all at least platform specific and often compiler specific as well.
Network libraries are the same, IO handling often as well (if you want more control than scanf and printf can handle).

gcc is no more generic than any other compiler, in fact it has a lot more platform specific libraries and some others and the documentation is often lacking.
In fact, Borland has always had the most standard compliant of all compilers on i386 platforms and a reasonably well marked custom library.
They also provide excellent documentation.

As to teachers being reluctant to change their material, often they aren't even aware they're using non-standard extensions, having learned C (or whatever) from the same book (Learning C in 21 days using Turbo C 1.5) they use to teach their students and never reading anything else.

thanks for the replies. i think i'm going to buy "C Programming Language : ANSI C Version, by Brian W. Kernighan, Dennis Ritchie".