meson: Use .dylib suffix if darwin

For Mac OS, GST_EXTRA_MODULE_SUFFIX should be set as '.dylib'.
Otherwise, GStreamer fails to load its plugins.

https://bugzilla.gnome.org/show_bug.cgi?id=793584
diff --git a/meson.build b/meson.build
index e909cfe..60a0a5d 100644
--- a/meson.build
+++ b/meson.build
@@ -83,6 +83,10 @@
 cdata.set_quoted('GST_PLUGIN_SCANNER_SUBDIR', libexecdir,
   description: 'libexecdir path component, used to find plugin-scanner on relocatable builds on windows')
 
+if host_system == 'darwin'
+  cdata.set_quoted('GST_EXTRA_MODULE_SUFFIX', '.dylib')
+endif
+
 if gst_version_nano > 0
     # Have GST_ERROR message printed when running from git
     cdata.set('GST_LEVEL_DEFAULT', 'GST_LEVEL_ERROR')