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.