Merge branches 'subdirs-simplify' and 'subdir-objects-pr10697' into maint

Merge branches 'subdirs-simplify' and 'subdir-objects-pr10697' into maint

* subdirs-simplify:
  subdirs: unify rules for "cleaning" and "normal" recursive targets
  tests: add a "demo" test on C support

* subdir-objects-pr10697:
  subdir-objects: improve "make mostlyclean" efficiency and flexibility
  tests: look for '.lo' rather than '.o' object when using Libtool with C++
  cosmetics: few typofixes in older ChangeLogs, suggested by "codespell.py"

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>