Mac OSX (Darwin) build support

Hi all,

as I have the opportunity to sit in front of a MacOSX machine, I thought it would be a good opportunity to try making Colobot build on this platform.

* First, OpenAL headers can't be found when using <AL/al.h> and the CMake finding script recommends to use "al.h" (0297cdf).
* system_other was apparently broken (87cd4fb).
* The biggest issue I have been having is that Mac OSX ships with an ancient (quasi-pre-historic) libstdc++, 4.2.1 (the latest GPL-2 version). This version:
 * doesn't support static map initialisation (90814ad)
 * has array in tr1 (374519d)
 * doesn't have std::string::append (799d455).
* The boost version as available in Mac Ports is compiled against this libstdc++, thereby enforcing the use of this stdlib. Colobot would definitely compile against the libc++ stdlib but boost is not available in that flavour on MacPorts as of now. I have added support in the CMake build infrastructure to enforce that (8c8444a).

Thanks for considering, cheers!

OdyX