add julia_version.h and also export version info for DLL

This was probably unintended.

Running `make` before committing, or using the `-p` option on `git add -p` will avoid issues like this.

I'm slowly beginning to realize that git has implemented submodules in a bad way, and that maybe we should download tarballs instead. Now that we have the  infrastructure to verify hashes, we don't even get the security benefit.