FetchContent_Declare unconditionally tries to clone SingleApplication
from GitHub, which breaks offline builds (e.g. Debian/Ubuntu pbuilder
with FETCHCONTENT_FULLY_DISCONNECTED=ON, Launchpad PPA builds).
If the SingleApplication submodule is already checked out in the
source tree, point FETCHCONTENT_SOURCE_DIR_SINGLEAPPLICATION at it so
FetchContent skips the network step entirely and reuses the local
copy. Falls back to the existing git clone behavior otherwise, so
this is a no-op for setups that don't vendor the submodule.
Laurent Trinques