gallium/va: set up symlinks in build-dir

Without this libva won't be able to find the driver without
LIBVA_DRIVER_NAME trickery, because the driver has a generic name.
But in the DRI case, even LIBVA_DRIVER_NAME won't do, because the driver
name needs to end with "_drv_video.so", which it doesn't.

So let's instead set up symlinks in the build-dir, like DRIL does.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40136>

meson.build

@@ -2099,6 +2099,8 @@ else
   prog_flex_cpp = prog_flex
 endif
 
+prog_ln = find_program('ln', required : false)
+
 _libunwind = get_option('libunwind') \
   .require(not with_platform_android, error_message : 'Android requires the use of the backtrace library, not libunwind')
 if host_machine.system() == 'darwin'

src/gallium/targets/dri/meson.build

@@ -75,7 +75,17 @@ if with_gallium_va
               [with_gallium_virgl, 'virtio_gpu'],
               [with_gallium_d3d12_video, 'd3d12']]
     if d[0]
-      va_drivers += '@0@_drv_video.so'.format(d[1])
+      name = '@0@_drv_video.so'.format(d[1])
+      va_drivers += name
+
+      if prog_ln.found()
+        custom_target(
+          name,
+          output : name,
+          command : [prog_ln, '-sf', libgallium_dri.full_path(), '@OUTPUT@'],
+          build_by_default : true,
+        )
+      endif
     endif
   endforeach
 

src/gallium/targets/dril/meson.build

@@ -131,7 +131,6 @@ foreach d : [[with_gallium_kmsro, [
 endforeach
 
 # This only works on Unix-like oses, which is probably fine for dri
-prog_ln = find_program('ln', required : false)
 if prog_ln.found()
   foreach d : dril_drivers
     custom_target(

src/gallium/targets/va/meson.build

@@ -92,7 +92,18 @@ else
               [with_gallium_virgl, 'virtio_gpu'],
               [with_gallium_d3d12_video, 'd3d12']]
     if d[0]
-      va_drivers += '@0@_drv_video.so'.format(d[1])
+      name = '@0@_drv_video.so'.format(d[1])
+      va_drivers += name
+
+      if prog_ln.found()
+        custom_target(
+          name,
+          output : name,
+          command : [prog_ln, '-sf', libva_gallium.full_path(), '@OUTPUT@'],
+          build_by_default : true,
+        )
+      endif
+
     endif
   endforeach