cmake: add option to check submodules

[abouvier]

Better than deleting .gitmodules manually :p

[Docteh]

What does this help with?

[abouvier]

When you non-recursively clone the repo to build yuzu, because you use system libraries instead of the externals. The build fails (because of the check_submodules_present function) if you don't delete .gitmodules before compiling.