MinGW: Work around lack of strtof_l for locale independent parsing

[amyspark]

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This PR is to catch a slipup in #1496 I've only found yesterday. MinGW-based compilers lack ucrtbase's definition of _strtof_l completely, and strtod_l is also missing from GCC, so neither are viable alternatives to achieve locale-independent floating point number parsing.

In this patch, I propose to apply a workaround similar to the one existing for macOS, that covers only this flavor of compilers.