make: fix GCC warnings

*** 306,317 ****
  extern  Boolean         stdout_stderr_same;
  extern  Dependency      suffixes;
  extern  Name            suffixes_name;
  extern  Name            sunpro_dependencies;
  extern  Boolean         target_variants;
! extern  char            *tmpdir;
! extern  char            *temp_file_directory;
  extern  Name            temp_file_name;
  extern  short           temp_file_number;
  extern  wchar_t         *top_level_target;
  extern  Boolean         touch;
  extern  Boolean         trace_reader;
--- 306,317 ----
  extern  Boolean         stdout_stderr_same;
  extern  Dependency      suffixes;
  extern  Name            suffixes_name;
  extern  Name            sunpro_dependencies;
  extern  Boolean         target_variants;
! extern  const char      *tmpdir;
! extern  const char      *temp_file_directory;
  extern  Name            temp_file_name;
  extern  short           temp_file_number;
  extern  wchar_t         *top_level_target;
  extern  Boolean         touch;
  extern  Boolean         trace_reader;