Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

Don't use FILE_ATTRIBUTE_NORMAL, it's a "magic" value for something
else.  To check if a file is a regular file, we need to check if it's
NOT a directory (or a device).

Replaces StatFile(), with a portable data object.