Add complier warning -wunused.

Cherry-picked from aomedia/master: 953f086c

Note: related fixes were already part of webm/nextgenv2.

Change-Id: I027a4f2a540af5a304b358ddbf293965b4211b9e

configure

@@ -615,20 +615,14 @@ process_toolchain() {
         check_add_cflags -Wvla
         check_add_cflags -Wimplicit-function-declaration
         check_add_cflags -Wuninitialized
-        check_add_cflags -Wunused-variable
+        check_add_cflags -Wunused
         check_add_cflags -Wsign-compare
         # Enabling the following warning for C++ generates some useless warnings
         # about some function parameters shadowing class member function names.
         # So, only enable this warning for C code.
         check_cflags "-Wshadow" && add_cflags_only "-Wshadow"
-        case ${CC} in
-          *clang*) ;;
-          *) check_add_cflags -Wunused-but-set-variable ;;
-        esac
         if enabled mips || [ -z "${INLINE}" ]; then
           enabled extra_warnings || check_add_cflags -Wno-unused-function
-        else
-          check_add_cflags -Wunused-function
         fi
     fi