make: unifdef for other OSes (undefined)

*** 323,336 ****
  extern  long int        hostid;
  
  /*
   * Declarations of system defined variables
   */
- #if !defined(linux)
  /* On linux this variable is defined in 'signal.h' */
  extern  char            *sys_siglist[];
- #endif
  
  /*
   * Declarations of system supplied functions
   */
  extern  int             file_lock(char *, char *, int *, int);
--- 323,334 ----
*** 404,423 ****
  #endif
  #if defined(DISTRIBUTED) || defined(MAKETOOL) /* tolik */
  extern  XDR             *get_xdrs_ptr(void);
  #endif
  extern  wchar_t         *getmem_wc(register int size);
- #if !defined(linux)
  /* On linux getwd(char *) is defined in 'unistd.h' */
  #ifdef __cplusplus
  extern "C" {
  #endif
  extern  char            *getwd(char *);
  #ifdef __cplusplus
  }
  #endif
- #endif
  extern  void            handle_interrupt(int);
  extern  Boolean         is_running(Name target);
  extern  void            load_cached_names(void);
  extern  Boolean         parallel_ok(Name target, Boolean line_prop_must_exists);
  extern  void            print_dependencies(register Name target, register Property line);
--- 402,419 ----