meson: add LIBGL_DRIVERS_PATH to the devenv

This allows using built dri OpenGL drivers with the `meson devenv`
command

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14826>

src/gallium/targets/dri/meson.build

@@ -118,6 +118,24 @@ foreach d : [[with_gallium_kmsro, [
   endif
 endforeach
 
+if meson.version().version_compare('>= 0.58')
+  # This only works on Unix-like oses, which is probably fine for dri
+  prog_ln = find_program('ln', required : false)
+  if prog_ln.found()
+    devenv.set('LIBGL_DRIVERS_PATH', meson.current_build_dir())
+
+    foreach d : gallium_dri_drivers
+      custom_target(
+        'devenv_@0@'.format(d),
+        input : libgallium_dri,
+        output : d,
+        command : [prog_ln, '-f', '@INPUT@', '@OUTPUT@'],
+        build_by_default : true,
+      )
+    endforeach
+  endif
+endif
+
 meson.add_install_script(
   install_megadrivers_py.path(),
   libgallium_dri.full_path(),