pcre: Skip winpthread dep

Seems to build just fine without it, maybe it was just using C++ threads
which mcfgthread provides?

pkgs/development/libraries/pcre/default.nix

···
   ]
     ++ optional (variant != null) "--enable-${variant}";
 
-  buildInputs = optional (stdenv.hostPlatform.libc == "msvcrt") windows.mingw_w64_pthreads;
-
   # https://bugs.exim.org/show_bug.cgi?id=2173
   patches = [ ./stacksize-detection.patch ];