message: Enable -Wredundant-decls (gcc) if available
diff: @@ -303,6 +303,7 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
  AX_CHECK_COMPILE_FLAG([-Wformat-security],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wformat-security"],,[[$CXXFLAG_WERROR]])
  AX_CHECK_COMPILE_FLAG([-Wthread-safety-analysis],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wthread-safety-analysis"],,[[$CXXFLAG_WERROR]])
  AX_CHECK_COMPILE_FLAG([-Wrange-loop-analysis],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wrange-loop-analysis"],,[[$CXXFLAG_WERROR]])
  + AX_CHECK_COMPILE_FLAG([-Wredundant-decls],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wredundant-decls"],,[[$CXXFLAG_WERROR]])
  ## Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
  ## unknown options if any other warning is produced. Test the -Wfoo case, and
  