Revert "create_symlinks: Make an exception for 1.10"

This reverts commit d4f7ebc8edccb11c511f5199d81b0a24af43716c.
campaignd: Fix abs() value truncation warning on clang 3.5
Remove `this` null pointer comparisons

It appears the standard specifies that `this` being null is undefined
behavior. Compilers may choose to optimize taking UB into account; in
particular, the null pointer check in question can be optimized out.
clang 3.5 has a new warning flag -Wtautological-pointer-compare which
warns against that particular case, among others.

http://llvm.org/releases/3.5.0/tools/clang/docs/ReleaseNotes.html

This fixes the following warnings with clang 3.5:

  src/actions/../game_data.hpp:66:5: error: 'this' pointer cannot be null in well-defined C++ code; comparison may be assumed to always evaluate to true [-Werror,-Wtautological-undefined-compare]
  src/actions/../game_data.hpp:74:5: error: 'this' pointer cannot be null in well-defined C++ code; comparison may be assumed to always evaluate to true [-Werror,-Wtautological-undefined-compare]
  src/actions/../game_data.hpp:130:5: error: 'this' pointer cannot be null in well-defined C++ code; comparison may be assumed to always evaluate to true [-Werror,-Wtautological-undefined-compare]
simplyfy win32 specific code

by unicode casting to std::wstring directly instead of using a
utf16::string temporary.
remove unused and undefined variable
fixup preprocessor symbols controlling SDL MIXER min version

Previously, the code that requires SDL_mixer v 1.2.12 was guarded
by #ifdef PANDORA, since this was the only platform intended to
use this. However, it turns out that OS X < 10.5 has a hard time
getting this library as well.

Instead of branching by platform, here we just check the version
defind by the SDL_mixer header, and include the code appropriate.

We rely on the scons / cmake / project file to fail any build which
should be done with the recent version.
Merge branch 'master' of https://github.com/wesnoth/wesnoth