configure: enable unused variable warnings

Only suppress unused function warnings, rather than supprressing all
unused-* warnings. Unused functions can still be seen with
--enable-extra-warnings.

Change-Id: Ibca20d859dbffedd76bd082ffe0fa685c3ac198e

configure

@@ -493,7 +493,7 @@ process_toolchain() {
         check_add_cflags -Wpointer-arith
         check_add_cflags -Wtype-limits
         check_add_cflags -Wcast-qual
-        enabled extra_warnings || check_add_cflags -Wno-unused
+        enabled extra_warnings || check_add_cflags -Wno-unused-function
     fi
 
     if enabled icc; then