Update support for OS/2

Update support for OS/2

Patches provided by KO Myung-Hun <komh78@gmail.com>

* NEWS: Add a note.
* AUTHORS: Add a new author.
* README.OS2: Updates to build instructions.
* src/dir.c (dir_contents_file_exists_p): Use a stack copy when
modifying a const string.
* src/job.c (construct_command_argv_internal): Ditto.
Reuse variables rather than re-defining them.
(exec_command): Cast a const string (we don't change it anyway).
* src/getopt.c (_getopt_initialize): Reference unused variables.
(_getopt_internal): Add block braces to quiet the compiler.
* src/main.c (main): Cast argument to child_execute_job().
* src/posixos.c (set_blocking): Reference unused variables.
* src/remake.c (f_mtime): Delete useless code.