use consistent names for the #include / #define header guard pattern.

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).

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