build: make depclean remove the userspace dependency files too

src/Makefile

@@ -526,17 +526,31 @@ endif
 
 # These rules clean things up.	'modclean' cleans files generated by 'modules'
 # (except that it doesn't remove the modules that were copied to rtlib)
-# 'clean' cleans everything but dependency files, and 'depclean' cleans them
-# too.
+# 'clean' removes the build products (including, via 'objects', the
+# dependency files), and 'depclean' removes the dependency files alone.
 modclean:
 	find -name '.*.cmd' -or -name '*.ko' -or -name '*.mod.c' -or -name '*.mod.o' | xargs rm -f
 	-rm -rf .tmp_versions
 	find . -name .tmp_versions |xargs rm -rf
 	-rm -f ../rtlib/*.ko
 	-rm -f ../rtlib/*.so
 
+# Dependency files land in two places: 'depends' for realtime under
+# BUILD_SYS=normal, and beside their objects as objects/**/*.d for
+# everything else, realtime included under BUILD_SYS=uspace, where
+# 'depends' is never created at all.  Both have to go.  Dropping them
+# forces no recompilation (nothing depends on a .d existing), but it does
+# clear stale prerequisites -- a dependency file naming a source that has
+# since been renamed or moved otherwise aborts the build with "No rule to
+# make target", and there is no way out of that short of a full
+# 'make clean'.
+# This is a find rather than '$(RM) $(DEPS) $(RTDEPS)' because the files
+# that break the build are precisely the ones whose source no longer
+# exists, so they are absent from $(DEPS); and $(DEPS) is only defined
+# when TRIVIAL_BUILD=no.  The pure-make form would not fix the bug.
 depclean:
 	-rm -rf depends
+	-find objects -name '*.d' -print0 2>/dev/null | xargs -0 -r rm -f
 
 clean: genclean depclean modclean
 genclean: