mingw fix: avoid "unused var" warning.

This is a backport of 19615bce64cd381a925bc3910120ac39ca918e7c to
fix bug 22838.

changes/bug22838_028

@@ -0,0 +1,5 @@
+  o Minor bugfixes (compilation, mingw, backport from 0.3.1.1-alpha):
+    - Backport a fix for an "unused variable" warning that appeared
+      in some versions of mingw. Fixes bug 22838; bugfix on
+      0.2.8.1-alpha.
+

src/test/test_switch_id.c

@@ -79,6 +79,7 @@ main(int argc, char **argv)
 #if defined(_WIN32)
   (void) argc;
   (void) argv;
+  (void) which_test;
 
   fprintf(stderr, "This test is not supported on your OS.\n");
   return 77;