Fixed silly NZBGet 9.1 & libpar2 issue

When I updated the Formula earlier, I only tested it once,
 and didn't think about the fact that I'd compiled libpar2
 myself on my system. NZBGet 9.1 included two patchs for
 libpar2, which won't be added upstream, as libpar2 upstream
 appears gone.

So, because I have no idea how to apply those patches only
 for NZBGet users, this disables the check in configure.

If I should instead add the patches to libpar2, please let me
 know.