59d5f692f removed cmake.deps/cmake/DownloadAndExtractFile.cmake and
support for USE_EXISTING_SRC_DIR. The Ubuntu nightly PPA still relies
on USE_EXISTING_SRC_DIR functionality since it can't access the network
during the build.
Supplying an empty value for ExternalProject_Add()'s URL value appears
to provide the needed mechanism to avoid re-downloading when the sources
are already present. This is undocumented behavior, though, so it may
break in the future.
Now, if USE_EXISTING_SRC_DIR is set, the ExternalProject's URL variable
is unset, preventing the download and erroring out if the source doesn't
actually exist.
61da959bb4