Windows XP has a large user base and there are a lot of companies using XP installations.
At this time! One year later ...
Its a matter of philosophy how to handle such cases. We all will be more and more driven by the market and lesser and lesser masters of our own decisions.
There are still tens of thousands of people using Windows 2000 ... in fact I still run into the occasional win98 box. But I think you misunderstand the problem here...
As I said before, the general CRT (C Run Time) library is not the problem. It can be statically linked which means that functions you use are copied right into your program. You can thus use a strictly C program from Pelles C 8.00 on a Windows 2000 system without worries.
The thing you have to be careful with is the Windows API... if you used functions that were included with Vista in your code, it's not going to run on an XP or Windows 2000 machine. You will get runtime errors telling you the functions don't exist. (Because, on that system, they don't).
This is why you will occasionally run into one of my libraries or addins that won't let you put it on a Win2000 machine... because, in order to get the desired functionality I had to use API calls that won't exist on your machine.
Of course the same problem exists with *every* piece of software currently out there. If they've used API calls that were first added for XP, Vista, 7 or 8, it's not going to run on your machine.
I agree this amounts to pressure to keep up... and to be honest I think that is a good thing. As I said before a lot of really good stuff has been invented and incorporated since Win2k.
Letting yourself fall behind by a couple of years is not likely to be a problem. Most programmers still write from the XP API, being fully aware that many of the newer functions are not present on those machines. However; slipping into a 15 year anachronism, in computer time, amounts to being positively medieval ... a true antique. Not only are you not going to be able to update your software, there are many things you simply cannot do... like flashdrives, external hard disks and a whole lot more.
Including OpenMP in PellesC is something of a departure. The complication is that it requires both hardware and OS support. So for you the simple answer comes down to one compiler flag you cannot use. You wanted compiler flags, there you have it...
But this here is Pelles decision!
Yes it is. I would have chosen otherwise but I'm not the boss. Implementing something like OpenMP shold be a function for an extra library, not part of the core compiler. But it is there and it's not a bad thing... so I don't see any problem using it.
As I said above... I see no problem for Pelle to Re-Enable XP and Win2000 support so long as he is careful to warn his users not to enable certain features on older systems.
Better still... POIDE could check the OS version and disable those flags from it's projects as a form of idiot proofing.