have_example_plugin.inc incorrect test

You may notice that the tests that use the example plugin are never
run. This is because the test in include/have_example_plugin.inc is
incorrect for mtr v2 (yes, this has been broken for a while).

This trivial patch fixes the include file to correctly detect that the
example plugin is around, and the tests run successfully.

The tests affected are (at least):
- main.plugin_load
- main.plugin_load_option
- main.bug46261
- main.plugin
- main.plugin_not_embedded
- rpl.rpl_plugin_load

I've not CC'd in Maria Developers as it appears to be fixed there,
albeit slightly differently.

--- a/mysql-test/include/have_example_plugin.inc	2012-01-20 11:35:48 +0000
+++ b/mysql-test/include/have_example_plugin.inc	2012-08-24 00:38:20 +0000
@@ -8,7 +8,7 @@ if (`SELECT @@have_dynamic_loading != 'Y
 #
 # Check if the variable EXAMPLE_PLUGIN is set
 #
-if (!$EXAMPLE_PLUGIN) {
+if (!$EXAMPLE_PLUGIN_OPT) {
   --skip Example plugin requires the environment variable \$EXAMPLE_PLUGIN to be set (normally done by mtr)
 }
 

-- 
Stewart Smith

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.11 (GNU/Linux)
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=bodA
-----END PGP SIGNATURE-----