curses: conditional resizeterm support.

* configure.ac (AC_CHECK_FUNCS): Check whether resizeterm is
implemented by the selected curses library.
* lcurses.c (Cresizeterm): If it is, use it, otherwis throw an
error to explain why not.
(curseslib): Add it to the toplevel curses functions.
Merge pull request #56 from gvvaughan/pull-request/curses-resizeterm-support

curses: conditional resizeterm support.