WIP: add_subdirectory(): add NAMESPACE support
See #16414
I just hacked this together, as a proof of concept. I don't do anything fancy: I just add the given NAMESPACE as a prefix to the subdirectory target names. I added a basic test case to the existing add_subdirectory() test suite. It seems to work, and there is no regression (tested on Linux only).
At the moment there is no validation of the NAMESPACE value.