Fix windows agent compile error/warnings #define ENOBUFS, ALERT_SYSTEM_ERR

The errno.h in some versions of MinGW do not have ENOBUFS defined, causing Travis CI windows_agent build to fail. This PR fixs that. 
Also, this PR gets rids of compile warnings regarding ALERT_SYSTEM_ERROR being redefined in rootcheck/rootcheck.h, which was also defined in /i686-w64-mingw32/include/winuser.h:4997