avoid unistd.h and sys/wait.h on Windows
avoid MSVC NovCTP compiler crashes due to {} field initialization
replace missing gmtime_r on Windows
add supported mkdir
avoid [[noreturn]] on MSVC
Remove default declaration of constructor.

This doesn't do anything really in this case and it makes problems on MSVC.
Use bool instead of void as return type in packaged_task.

MSVC can not handle void as return type, so we use a dummy bool instead.
Fixes #32.