I will argue that providing Unix-based tools is an important goal. And with autoconf and the usual Unix-based development tools, porting between various flavors, from Solaris to the different Linux distros to OS X, and even Cygwin on Windows (which is what supports the Xilinx tools, after all), should not be all that painful.

You run into issues when you have to support hardware such as JTAG dongles through device drivers, but at least if you do a USB device, you can take advantage of OS-supported class drivers to ease that pain.

Supporting several different platforms means testing on all of those platforms which will be the biggest issue for you.

-a