xorg.xorgserver: fix darwin build

disable libunwind detection as the configure step will detect the
libunwind pkg-config stub but then the code will uses the unimplemented
function unw_strerror.

the pkg-config stub was added in ec40b04

Author: paparodeo

pkgs/servers/x11/xorg/overrides.nix

@@ -890,6 +890,7 @@ self: super:
         configureFlags = [
           # note: --enable-xquartz is auto
           "CPPFLAGS=-I${./darwin/dri}"
+          "--disable-libunwind" # libunwind on darwin is missing unw_strerror
           "--disable-glamor"
           "--with-default-font-path="
           "--with-apple-application-name=XQuartz"