configure: Add compiler warning -Wunused-but-set-parameter.

[Gerald Pfeifer]

There was only one instance where this triggered in all of Wine, and that 
I fixed a week or two ago, so this can now go in.

GCC 4.6 and above support it, and it's not enabled by any of the existing 
warnings (such as -Wall).

Gerald


diff --git a/configure.ac b/configure.ac
index 51d0533..82b1783 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1707,6 +1707,7 @@ then
   WINE_TRY_CFLAGS([-Wdeclaration-after-statement])
   WINE_TRY_CFLAGS([-Wstrict-prototypes])
   WINE_TRY_CFLAGS([-Wtype-limits])
+  WINE_TRY_CFLAGS([-Wunused-but-set-parameter])
   WINE_TRY_CFLAGS([-Wwrite-strings])
 
   dnl Check for noisy string.h