fix undefined variable warnings for meson build

data/session/meson.build

@@ -1,5 +1,6 @@
 desktop_config = configuration_data()
 desktop_config.set('prefix', dir_prefix)
+desktop_config.set('VERSION', e_version_rev)
 e_desktop = configure_file(input        : 'enlightenment.desktop.in',
                output       : 'enlightenment.desktop',
                configuration: desktop_config

data/units/meson.build

@@ -1,8 +1,10 @@
+unit_config = configuration_data()
+unit_config.set('prefix', dir_prefix)
 if get_option('systemd') == true
   configure_file(input        : 'enlightenment.service.in',
                  output       : 'enlightenment.service',
                  install      : true,
                  install_dir  : systemd_unitdir,
-                 configuration: config_h
+                 configuration: unit_config
                 )
 endif