[Build] Bundled "extern" sources in tarball broken

[StefanBruens]

af_dep_check_and_populate calls FetchContent_Populate, which tries to update the sources, but fails as the shipped sources do not contain the required .git subdirectory.

Description

AF uses the multi-argument form of FetchContent_Populate:

https://cmake.org/cmake/help/latest/module/FetchContent.html#command:fetchcontent_populate

This form has a number of key differences to that where only <name> is provided:

• ...
• The FETCHCONTENT_FULLY_DISCONNECTED and FETCHCONTENT_UPDATES_DISCONNECTED cache variables are ignored.

AF should use FetchContent_Declare + FetchContent_MakeAvailable instead, so a fully disconnected build from the source tarball can actually work.

[StefanBruens]

See also arrayfire/forge#236

[9prady9]

The tarball generation is automated and the files being included was tested before we finalised on this particular workflow. If for some reason, this is failing it needs to be looked into.