Properly disable LUA_USE_DLOPEN for bootstrap flua

flua does have some specific bits that will include luaconf.h, but the
definition of LUA_USE_DLOPEN for those won't matter. This belongs in liblua
instead.

To expand on my previous commit, which was a little sparse with details,
it's not really safe to allow LUA_USE_DLOPEN with host lib paths being used.
The host system could have an entirely different lua version and this could
cause us to crash and burn.

If we want to revive this later, we need to make sure to define c module
paths inside OBJDIR that are compiled against whatever version we've
bootstrapped.

Pointy hat: kevans