fix segfault when cleaning up MixChunks on windows

We built the libSDL binaries several years ago, and so it was built
against an older runtime library (msvcrt.dll).
Now we build SDL itself against msvcr110.dll for example, and therefore
allocate on another heap than the free() is taking place, which happens
in libSDL_mixer itself.
See: http://stackoverflow.com/questions/23257226/sdl-mixer-mix-freechunk-crashing-on-sample-created-in-memory
dev release 2.543_1
updated Alien::SDL's version to get an OpenBSD/pthread fix, dev release 2.543_2
fix required version of dependency Alien::SDL
another attempt to correctly update dependency version of Alien::SDL
release 2.544
tests: remove old (and wrong) Hurd workaround

Other than being no more applicable with recent Hurd versions
(e.g. 0.5), it was working around a bug in TestTool::init, not properly
handling failures in SDL::init.

Basically it reverts commit 4931c49d1dfdc11aaf86d85db734104a37bef538.