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
diff --git a/Makefile b/Makefile
index 9acbf2b..7d39cfd 100644
--- a/Makefile
+++ b/Makefile
@@ -597,7 +597,7 @@
 
 BUILD_OPTIONS   += $(call ignore_implicit,USE_LUA)
 OPTIONS_CFLAGS  += -DUSE_LUA $(if $(LUA_INC),-I$(LUA_INC))
-LUA_LD_FLAGS := $(if $(LUA_LIB),-L$(LUA_LIB))
+LUA_LD_FLAGS := -Wl,--export-dynamic $(if $(LUA_LIB),-L$(LUA_LIB))
 ifeq ($(LUA_LIB_NAME),)
 # Try to automatically detect the Lua library
 LUA_LIB_NAME := $(firstword $(foreach lib,lua5.3 lua53 lua,$(call check_lua_lib,$(lib),$(LUA_LD_FLAGS))))