Bump hard SDL requirement to SDL 1.2.10 and soft to 1.2.15

SDL 1.2.15 contains important bug fix for Wesnoth:
  https://bugzilla.libsdl.org/show_bug.cgi?id=894
  http://forums.wesnoth.org/viewtopic.php?f=5&t=41237

Further discussion:
 https://github.com/wesnoth/wesnoth/pull/348
Merge pull request #348 from techtonik/sdl1215

Require SDL 1.2.15, which has important bug fix for Wesnoth