Display an error if cmake fails to find git during the submodules step.
Display an error if cmake fails to find git during the submodules step.

On some setup such as mine, git does not exist in $PATH

gdeda/openomf,omf2097/openomf,gdeda/openomf,pmjdebruijn/openomf,omf2097/openomf,pmjdebruijn/openomf,omf2097/openomf,pmjdebruijn/openomf,gdeda/openomf