xserver: replace XKB_BINDIR with compile time option

---

Using the configure option relieves us of the patch and passing the path
via the env var in many places. Also the env var may not be inherited
when components like gdm spawn new sessions.

pkgs/tools/admin/tigervnc/default.nix

@@ -20,7 +20,6 @@ stdenv.mkDerivation rec {
 
   patchPhase = ''
     sed -i -e 's,$(includedir)/pixman-1,${if stdenv ? cross then pixman.crossDrv else pixman}/include/pixman-1,' unix/xserver/hw/vnc/Makefile.am
-    sed -i -e '/^$pidFile/a$ENV{XKB_BINDIR}="${if stdenv ? cross then xorg.xkbcomp.crossDrv else xorg.xkbcomp}/bin";' unix/vncserver
     sed -i -e '/^\$cmd \.= " -pn";/a$cmd .= " -xkbdir ${if stdenv ? cross then xkeyboard_config.crossDrv else xkeyboard_config}/etc/X11/xkb";' unix/vncserver
     fontPath=
     for i in $fontDirectories; do