[widelands/f16] (6 commits) ...Rebase to new upstream release build17

[Nils Philippsen]

Summary of changes:

  2e2fe1f... Rebuild for new libpng (*)
  f201031... Fix building with libpng-1.5 (*)
  3e661b1... Add comment with upstream bug url for libpng-1.5 compat (*)
  fb53ae4... rebuild for gcc 4.7 (*)
  8dd68d3... - Rebuilt for c++ ABI breakage (*)
  f70d11b... Rebase to new upstream release build17 (*)

(*) This commit already existed in another branch; no separate mail sent