Conditionally test if Linux has epoll and eventfd

Test if epoll exists by testing __NR_epoll_create and conditionally include epoll.h and eventfd.h

linux-x86-default/inc/bitos.h:412:30: sys/epoll.h: No such file or directory
Set _FILE_OFFSET_BITS conditionally based on __USE_FILE_OFFSET64.

Fixes building on Linux 2.4 which lacks sendfile64.
Message:

/usr/include/sys/sendfile.h:26:3: #error "<sys/sendfile.h> cannot be used with _FILE_OFFSET_BITS=64"