autoconf: Added the -Wunused-but-set-variable to the default compile to help…

autoconf: Added the -Wunused-but-set-variable to the default compile to help avoid commiting more unused variable but set value.  Please report if you do have problem with it.

configure.in

@@ -583,6 +583,17 @@ if test x"$ac_cv_gcc_declaration_after_statement" = xyes; then
 fi
 CFLAGS="$saved_CFLAGS"
 
+saved_CFLAGS="$CFLAGS"
+AC_CACHE_CHECK([whether compiler supports -Wunused-but-set-variable], [ac_cv_gcc_unused_but_set_variable], [
+CFLAGS="$CFLAGS -Wunused-but-set-variable"
+AC_TRY_COMPILE([],[return 0;],[ac_cv_gcc_unused_but_set_variable=yes],[ac_cv_gcc_unused_but_set_variable=no])
+])
+AC_MSG_RESULT($ac_cv_gcc_unused_but_set_variable)
+if test x"$ac_cv_gcc_unused_but_set_variable" = xyes; then
+    APR_ADDTO(SWITCH_ANSI_CFLAGS, -Wunused-but-set-variable)
+fi
+CFLAGS="$saved_CFLAGS"
+
 AC_C_BIGENDIAN(AC_DEFINE([SWITCH_BYTE_ORDER],__BIG_ENDIAN,[Big Endian]),AC_DEFINE([SWITCH_BYTE_ORDER],__LITTLE_ENDIAN,[Little Endian]))
 
 # Checks for integer size