[lua] update to 5.3.4

[codicodi]

Also add (off by default) option to compile as C++ and bake dllimport to headers in dynamic builds.

[ras0219-msft]

Could you briefly summarize why you would or wouldn't use COMPILE_AS_CPP in a comment in the CMakeLists.txt? It's especially opaque to me why we don't install lua.hpp when compiling as C++ -- I'd assume it should be the opposite.

[codicodi]

It is obvious once you look at its contents :)

// lua.hpp
// Lua header files for C++
// <<extern "C">> not supplied automatically because Lua also compiles as C++

extern "C" {
#include "lua.h"
#include "lualib.h"
#include "lauxlib.h"
}

As for why compile as C++ in the first place, see http://stackoverflow.com/questions/13560945/c-and-c-library-using-longjmp.
I'll add a comment about this

[ras0219-msft]

Thanks! I extended the comment a little more with some notes for users who might come after us.

In the future, it might make sense to synthesize a lua.hpp that just includes the other headers without a wrapping extern "C". This would make it possible for C++ libraries to just always include lua.hpp and work correctly regardless of whether COMPILE_AS_CPP is on or off.