allow newer compilers

[JJones780]

See Issue #2
It turns out the git submodule update wasn't getting the newest libsvflow
This required a change to the Readme instructions.
Once that was done there were a few directory issues with two CMakeLists.txt files.
No code changes were required.

Considerations for the README changes:
You might not want to pull the most recent submodule (in the future) - I didn't research how to set the version that the existing git submodule update --recursive was retrieving.

[Granjow]

Thanks @JJones780 for the PR.

The status of a submodule is registered in Git by its commit hash. If you update the commit in the submodule, you can git add the submodule in the main repo, and it will from then on point to the new commit hash.

This is a way to ensure that the submodule is at a working version, which updating to the lastest version with --remote does not guarantee because that may contain breaking changes. It works in this case, but only by accident :)

[Granjow]

Please update the submodule instead (see comment)

[Granjow]

PS: For some reason the default branch was changed, I changed it back to master, please update your PR accordingly. Thanks :)

[JJones780]

commit f91: I removed the git submodule's --remote option in the build instructions.
commit 9c8: I followed the instructions here to sync the submodule.
Re-cloned this fork and all compiled well ( using just git submodule update --init )