/* Mingw32 defaults to globing command line
 * So we turn it off like this:
 */
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The comments in runperl.c provide some additional explanation:

/* Mingw32 defaults to globing command line 
 * This is inconsistent with other Win32 ports and 
 * seems to cause trouble with passing -DXSVERSION=\"1.6\" 
 * So we turn it off ....
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From that, we gather that one reason for setting _CRT_glob to 0 was to bring perl into line with perls built using a Microsoft compiler.
IMO, this is no longer a valid reason. (I'm not even sure that it was ever a valid reason.)

But then there's also a vague reference to a problem with passing --DXSVERSION=\"1.6\" when _CRT_glob is set to 1.
Do we know what that's about ?
Do we know that it's no longer an issue ?
How does make test fare ?

There are other discrepancies: while in mingw-w64 _CRT_glob is a boolean in mingw.org it's a bitfield.. so good luck.

A couple of weeks ago I tried building perl with mingw.org's gcc-9.2.0, but got stumped by their recent decision to change sizeof(FD_SET) to "1". Both Microsoft (at least up to VS 2019) and mingw-w64 compilers have sizeof(FD_SET) == 260 (or 520 for mingw-w64 64-bit compilers). I don't know why mingw.org changed that spec, but these days they do seem to have quite a knack for making things difficult.
To put it bluntly, I regard mingw.org as currently irrelevant - though that's perhaps unfair.

Cheers,
Rob