#71 Make public headers platform-independent

[Andrei Kapustin]

It would be very nice if the public (aka "CARL_XXX.hpp") headers had none of the "#if (CARL_OS_THIS)" or "#if (CARL_TOOLCHAIN_THAT)" stuff. Apart from the "CARL_Platform.hpp", which is, anyway, platform-dependent, all other headers shall not be. Any deviation from this would-be-rule is OK if documented.