Allow setting -I and -L via env variables

Now LIBS and CFLAGS can be used to provide extra compilation arguments.
Fix double free bug for Capp.offline on an IO

Both the ruby IO and capp close the IO when garbage collected.  This will
lead to a double-free and possible segmantation fault.

Now capp dups the file descriptor and creates a new stream (FILE *) for the
file descriptor that libpcap can safely close without closing the ruby stream
(and vice-versa).