Use FindCurses to find curses libraries

- Some systems have the FindCurses.cmake module to find
  the curses/ncurses libraries using find_package(). And
  in some CheckLibraries is not very reliable, so as
  fallback FindCurses is now used if no other option
  works.

src/CMakeLists.txt

@@ -35,7 +35,8 @@ else()
   if (HAVE_LIBCURSES)
     target_link_libraries(nvim curses)
   else()
-    message(FATAL_ERROR "can't find something resembling -ltermcap")
+    find_package(Curses REQUIRED)
+    target_link_libraries(nvim ${CURSES_LIBRARIES})
   endif()
 endif()