In the Unix world, "portability" refers to porting to different Unix versions. For a GNU program, this kind of portability is desirable, but not paramount.
The primary purpose of GNU software is to run on top of the GNU kernel, compiled with the GNU C compiler, on various types of CPU. The amount and kinds of variation among GNU systems on different CPUs will be comparable to the variation among Linux-based GNU systems or among BSD systems today. So the kinds of portability that are absolutely necessary are quite limited.
But many users do run GNU software on non-GNU Unix or Unix-like systems. So supporting a variety of Unix-like systems is desirable, although not paramount.
The easiest way to achieve portability to most Unix-like systems is to use Autoconf. It's unlikely that your program needs to know more information about the host platform than Autoconf can provide, simply because most of the programs that need such knowledge have already been written.
Avoid using the format of semi-internal data bases (e.g., directories)
when there is a higher-level alternative (readdir).
As for systems that are not like Unix, such as MSDOS, Windows, the Macintosh, VMS, and MVS, supporting them is usually so much work that it is better if you don't.
The planned GNU kernel is not finished yet, but you can tell which facilities it will provide by looking at the GNU C Library Manual. The GNU kernel is based on Mach, so the features of Mach will also be available. However, if you use Mach features, you'll probably have trouble debugging your program today.
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