Don't use off64_t/_wfopen on MinGW32 in C++11 mode

Unfortunately, standard headers on MinGW32 insist on undefining off64_t
and _wfopen extensions if __STRICT_ANSI__ is true (e.g. C++11 mode). This
leads to compilation errors since b7a1fecc started to use _wfopen in strict
mode. That change erroneously checked GCC version - however, the version
itself is irrelevant; the actual criteria is whether mingw64 runtime is
used.

off64_t is not useful on MinGW32 since we only need it to open large files
on 64-bit platforms; unfortunately, the lack of _wfopen means we won't be
able to support wide-char paths on Windows for MinGW32.

Fixes #24.