LuaJIT ignores conflicting typedefs

[Florian Weimer <fw@...>]

The following prints "ctype<int>", that is, the first typedef takes
precedence:

local ffi = require "ffi"
ffi.cdef [[
typedef int foo;
typedef long foo;
]]
print(ffi.typeof("foo"))

This is on amd64 (using LuaJIT 2.0.0-beta9), so the types are actually
quite distinct.