[retro-gtk] build: Fix RETRO_PLUGIN_PATH

[Adrien Plazas <aplazas src gnome org>]

commit ed46511e4b77c36c90fcfdf50270dd5999f62217
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Sun Nov 5 14:40:02 2017 +0100

    build: Fix RETRO_PLUGIN_PATH

 meson.build           |    5 +++--
 retro-gtk/meson.build |    1 +
 2 files changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/meson.build b/meson.build
index dc9e154..5f560f7 100644
--- a/meson.build
+++ b/meson.build
@@ -5,8 +5,9 @@ project('retro-gtk','c',
 
 gnome = import('gnome')
 
-libdir = get_option ('libdir')
-libretrodir = join_paths (libdir, 'libretro')
+prefix = get_option('prefix')
+libdir = join_paths(prefix, get_option('libdir'))
+libretrodir = join_paths(libdir, 'libretro')
 
 confinc = include_directories('.')
 srcinc = include_directories('retro-gtk')
diff --git a/retro-gtk/meson.build b/retro-gtk/meson.build
index 1840921..7b86739 100644
--- a/retro-gtk/meson.build
+++ b/retro-gtk/meson.build
@@ -76,6 +76,7 @@ configure_file(
 retro_gtk_c_args = [
   '-DRETRO_GTK_COMPILATION',
   '-DG_LOG_DOMAIN="Retro"',
+  '-DRETRO_PLUGIN_PATH="@0@:@1@"'.format(libretrodir, libdir),
 ]
 
 retro_gtk_deps = [