This check is used to set $EXEXT, which is unused, but it also sets
$ac_exeext which is used internally for various tests, including
AC_TRY_LINK.

Interestingly, setting $ac_exeext to a non empty value bypasses the only
portability issue with MinGW that automatically adds a `.exe` suffix if no
extension is given. So Always set `.out` as extension, as it's only used
internally.

Differential Revision: https://phabricator.services.mozilla.com/D207001