Add note about need libevent for build examples

(May be also add check if libev is available because the build fail :
In file included from client.cc:38:0:
client.h:38:16: fatal error: ev.h: No such file or directory

Add ax_cxx_compile_stdcxx(.m4) file for build with Ubuntu 16.04

to avoid
./configure: line 17124: syntax error near unexpected token AX_CXX_COMPILE_STDCXX(14, noext, optional)'

When ./configure

Also fix the bug that pkt_num was uint32_t.