Android NDK does not have a `struct timespec` in its `struct stat`
for nanosecond support, instead it has a single nanosecond member inside
the struct stat itself.  We will use that and use a macro to expand to
the `st_mtim` / `st_mtimespec` definition on other systems (much like
the existing `st_mtime` backcompat definition).

win32: drop xp support in WideCharToMultiByte

Windows defines `timeval` with `long`, which we cannot
sanely cope with.  Instead, use a custom timeval struct.

git_index_entry__init_from_stat: set nsec fields in entry stats

This is where portable git stores the global configuration which we can
use to adhere to it even though git isn't quite installed on the system.