mingw: use ANSI or Unicode functions explicitly

For many Win32 functions, there actually exist two variants: one with
the `A` suffix that takes ANSI parameters (`char *` or `const char *`)
and one with the `W` suffix that takes Unicode parameters (`wchar_t *`
or `const wchar_t *`).

Let's be precise what we want to use.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho