MDEV-31963 cmake: fix libfmt usage

`fmt::detail::make_arg` does not accept temporaries, so the code snippet
checking system libfmt needs to be adjusted.
Signed-off-by: Ruoyu Zhong <zhongruoyu@outlook.com>

cmake/libfmt.cmake

@@ -33,8 +33,9 @@ MACRO (CHECK_LIBFMT)
      #include <fmt/format-inl.h>
      #include <iostream>
      int main() {
+       int answer= 42;
        fmt::format_args::format_arg arg=
-         fmt::detail::make_arg<fmt::format_context>(42);
+         fmt::detail::make_arg<fmt::format_context>(answer);
          std::cout << fmt::vformat(\"The answer is {}.\",
                                    fmt::format_args(&arg, 1));
      }" HAVE_SYSTEM_LIBFMT)