Attempt to complile Python/dynload_win.c by MinGW fails
due to static reimplementation of strcasecmp function in this file:

---
/* Case insensitive string compare, to avoid any dependencies on particular
   C RTL implementations */

static int strcasecmp (char *string1, char *string2)
{
    int first, second;

    do {
        first  = tolower(*string1);
        second = tolower(*string2);
        string1++;
        string2++;
    } while (first && first == second);

    return (first - second);
}
---

And this reimplementation clashed with native declaration of strcasecmp()
which one is a part of MinGW runtime

So suggested patch (for 3.5.2 and 2.7.12)
just disables static reimplementation of strcasecmp for MinGW

Why not replace it entirely with stricmp? Does it behave differently?

New changeset 05f2d84cae4ba1ff15b7a1d0347305393f4bdcc5 by Serhiy Storchaka (Minmin Gong) in branch 'master':
bpo-28269: Replace strcasecmp with system function _stricmp. (GH-13095)
https://github.com/python/cpython/commit/05f2d84cae4ba1ff15b7a1d0347305393f4bdcc5

New changeset 920ec4b7763d64b3742d7ddd339ad11cdbec62e9 by Miss Islington (bot) in branch '3.8':
bpo-28269: Replace strcasecmp with system function _stricmp. (GH-13095)
https://github.com/python/cpython/commit/920ec4b7763d64b3742d7ddd339ad11cdbec62e9

Thank you for your PR Minmin. It was decided to not backport it to 3.7.