YCMEPHelper: Cleanup
ExternalProject: Add SCM_DISCONNECTED argument to ExternalProject_Add

If SCM_DISCONNECTED is set, the update step is not executed
automatically when building the main target. The update step can still
be added as a step target and called manually. This is useful if you
want to allow to build the project when you are disconnected from the
network (you might still need the network for the download step).

This is disabled by default. The directory property EP_SCM_DISCONNECTED
can be used to change the default value for all the external projects
in the current directory and its subdirectories.
YCMEPHelper: Update ExternalProject sha1sum
YCMEPHelper: Enable SCM_DISCONNECTED for all the projects

The update step must now be called manually.
Merge branch 'SCM_DISCONNECTED'