MinGW support

[andrey-starodubtsev]

• fixed: format specifiers like PRIu32 or PRIdMAX are defined in standard C header inttypes.h

• fixed: _snprintf is a function introduced by MicroSoft, which isn't fully compliant with snprintf from C99 (it doesn't support %zu, for example). snprintf/vsnprintf are available in MinGW though

• fixed: _GNU_SOURCE was added in fadb26aa, Aug 23 2017 in attempt to enable declaration of GNU extension vasprintf. But later in a06b1b4d and 6bdd8c0a more correct way of handling this function was added - via definition of subsitution if it's absent (e.g. for Solaris); so enforcing _GNU_SOURCE isn't necessary anymore, and it breaks MinGW build.

[mouse07410]

Thank you fur submitting. Let me see the GH Actions result better merging.