stub termcap, add extern "C"

stub termcap, add extern "C"

Fixes linking an --enable-build-with-cxx build on mingw:

 ../readline/terminal.c:278: undefined reference to `tgetnum'
 ../readline/terminal.c:297: undefined reference to `tgetnum'
 ../readline/libreadline.a(terminal.o): In function `get_term_capabilities':
 ../readline/terminal.c:427: undefined reference to `tgetstr'
 ../readline/libreadline.a(terminal.o): In function `_rl_init_terminal_io':
 [etc.]

gdb/ChangeLog:
2015-03-16  Yuanhui Zhang  <asmwarrior@gmail.com>
	    Pedro Alves  <palves@redhat.com>

	* gdb_curses.h (tgetnum): Mark with EXTERN_C.
	* stub-termcap.c (tgetent, tgetnum, tgetflag, tgetstr, tputs)
	(tgoto): Wrap with extern "C".