BUG/MINOR: lua: can't load external libraries

BUG/MINOR: lua: can't load external libraries

Libraries requires the export of embedded Lua symbols. If a library
is loaded by HAProxy or by an Lua program, an error like the following
error raises:

   [ALERT] 085/135722 (7224) : parsing [test.cfg:8] : lua runtime error: error loading module 'test' from file './test.so':
        ./test.so: undefined symbol: lua_createtable

This patch modify the Makefile, and allow exports of the Lua symbols.

This patch must be backported in version 1.6