ncurses 6.1 - patch 20181215

+ several fixes for gcc8 strict compiler warnings.
+ fix a typo in comments (Aaron Gyes).
+ add nsterm-build309 to replace nsterm-256color, assigning the latter
  as an alias of nsterm, to make mouse work with nsterm-256color -TD
+ base gnome-256color entry on "gnome", not "vte", for consistency -TD
+ updates for configure macros from work on tin and xterm:
  + CF_GNU_SOURCE, allow for Cygwin's newlib when checking for the
    _DEFAULT_SOURCE symbol.
  + CF_VA_COPY, add fallback check if neither va_copy/__va_copy is
    supported, to try copying the pointers for va_list, or as an array.
    Also add another fallback check, for __builtin_va_copy(), which
    could be used with AIX xlc in c89 mode.