memcached: remove hard-coded library paths

Instead of specifying hard-coded path of libevent library in usr.manifest
(which of course changes on different machines), compile with "-levent"
and use the ldd command to figure out 1) which shared library the executable
expects to load, and 2) where it is found on the host machine, and add this
as a line to the usr.manifest.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>