backend/Makefile: Fix handling of built sources
This reverts commit 939d62f8 which broke make dist
when it was run locally in a CI container. Having $(srcdir)/
as part of the Makefile target is necessary for this particular case.
This otherwise tweaks the handling of built sources in the backend/ directory, ensuring that they are regenerated when necessary, and that make clean
and make distclean
only remove them when it is appropriate.