Don't #undef stat and #define stat(). stat is one of those unfortunate

Don't #undef stat and #define stat(). stat is one of those unfortunate
identifiers for which there is both a struct and a function. MinGW uses
a #define for setting struct stat to be the right struct, so doing #undef
stat to clear the way for a #define for stat() the function
inadvertantly clobbers the selected stat struct. To avoid this, we
define la_stat() to _la_stat on Windows and stat() elsewhere, and then
use la_stat() instead of stat().