fix aio,buffile compilation under windows

link with -lpotion
add 3rd/libuv/Makefile.am rule to do git submodule update --init
do not link -z on cygwin
improve INSTALL.md for windows
more mingw32 fixes for aio and buffile

f*lockfile are not defined on mingw
aio needs -lpthread -lws2_32 -lpsapi