Merge branch '2944_cleanup'

Merge branch '2944_cleanup'

# By Andrew Borodin (10) and Slava Zanko (1)
# Via Slava Zanko
* 2944_cleanup:
  Avoid compiler errors like:  error: variable 'xxx' set but not used [-Werror=unused-but-set-variable]
  Clarify check of C compiler flags.
  Enable -Wunused-but-set-variable flag by default.
  Clarify usage of use_internal_edit variable
  Merge src/filemanager/option.[ch] to src/filemanager/boxes.[ch].
  Remove irrelevant comments about file_date() function.
  Avoid C++-style comments.
  (mc_global_t::shell): new member to store user's shell
  (subshell_prompt): changed to GString.
  (read_subshell_prompt): return gboolean instead of int.
  Ticket #2944: code cleanup before 4.8.8 release.