Editline v1.15.2

Bug fixes and minor feature creep in pkg-config support.

Changes

• Prevent mangling of symbols when linking with C++. Patch courtesy of
  Jakub Pawlowski
• Add libeditline.pc for pkg-config

Fixes

• Do not assume a termcap library exists, check for tgetent() in
  curses, ncurses, tinfo and termcap libraries
• Call tty_flush() when user calls rl_forced_update_display()
  to prevent screen becoming garbled. Patch by Jakub Pawlowski