Improve checking for presence of submodule

As libdivecomputer and subsurface are submodules, they
would always be present as folder on git cloning. So, on
new install, this would always be false and the submodules
would not be updated. But on git clone, these folders
remain empty. So, if one checks the existence of some file
that should obviously be present, all would be fine. Hence,
the check is modified.

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>