mesa: fix libGL searching for libudev

Close #2784, #2731.

I don't have a clue if using our libudev is always correct on non-nixos,
or if using /run/current-system/sw/lib/libudev would be better...

pkgs/development/libraries/mesa/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, pkgconfig, intltool, flex, bison, autoreconfHook
+{ stdenv, fetchurl, pkgconfig, intltool, flex, bison, autoreconfHook, substituteAll
 , python, libxml2Python, file, expat, makedepend
 , libdrm, xorg, wayland, udev, llvm, libffi
 , libvdpau, libelf
@@ -45,7 +45,11 @@ stdenv.mkDerivation {
     ./glx_ro_text_segm.patch # fix for grsecurity/PaX
    # TODO: revive ./dricore-gallium.patch when it gets ported (from Ubuntu),
    #  as it saved ~35 MB in $drivers; watch https://launchpad.net/ubuntu/+source/mesa/+changelog
-  ];
+  ] ++ optional stdenv.isLinux
+      (substituteAll {
+        src = ./dlopen-absolute-paths.diff;
+        inherit udev;
+      });
 
   # Change the search path for EGL drivers from $drivers/* to driverLink
   postPatch = ''

pkgs/development/libraries/mesa/dlopen-absolute-paths.diff

@@ -0,0 +1,13 @@
+diff --git a/src/loader/loader.c b/src/loader/loader.c
+index 666d015..4d7a9be 100644
+--- a/src/loader/loader.c
++++ b/src/loader/loader.c
+@@ -101,7 +101,7 @@ static void *
+ udev_dlopen_handle(void)
+ {
+    if (!udev_handle) {
+-      udev_handle = dlopen("libudev.so.1", RTLD_LOCAL | RTLD_LAZY);
++      udev_handle = dlopen("@udev@/lib/libudev.so.1", RTLD_LOCAL | RTLD_LAZY);
+ 
+       if (!udev_handle) {
+          /* libudev.so.1 changed the return types of the two unref functions