warning about conversion type character 'L' in printf

This code

    long double phi = ...;
    printf( "phi %20.14Le\n", phi);

results in this warning

    unknown conversion type character 'L' in format [-Wformat=]

when compiling with MinGW-gcc 8.2.0, -C99.

 From https://en.cppreference.com/w/cpp/io/c/fprintf
I understand that the length modifier 'L' is standard
(even pre 99), and that it is compatible with 'e'.

No warning when compiling the same code with gcc under Linux.

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