MFC r256842:

  Fix 'make delete-old-libs' and 'make check-libs' to delete .debug
  files created by WITH_DEBUG_FILES. Also cleanup .symbols files from
  the period between r244236 when .symbols were supported and r251512
  when they were renamed to .debug.

  Only propose to delete a .debug file if the corresponding library
  itself was deleted already.

Approved by:	bapt
Approved by:	re (glebius)