Haiku: Several fixes to platform module

* Do not define BEOS anymore (this includes workarounds which we don't
  need most of the time in Haiku, so we prefer opt-in IF(HAIKU) in the
  cmake files instead).
* On the other hand, do define UNIX (we are trying to be compliant) and
  HAIKU (there is still a number of things we don't do like the
  average UNIX clone)
* Do not use UnixPaths, as our filesystem hierarchy isn't anything like
  what it expects.
* Do not use -nostart, which the compiler doesn't know about anymore.
  This used to be an Haiku extension to gcc, and is equivalent to
  -shared which is the default gcc option.
* While "dl" functions are provided in libroot, this is always
  implicitly linked so there is no need to tell cmake about it.
* Forcing position-independent code is not needed, so remove it.
* On the other hand, include appropriate linker options for executables
  and shared libraries.
* Support for the two available compilers in Haiku (gcc2 and gcc4) and
  pick the right headers and libraries according to the currently
  selected one.
* With the adoption of the package manager, the directory layout was
  changed. Tell cmake where to look for header files and libraries.
* As we don't define BEOS anymore, enable the workaround we still need
  for HAIKU as well. This is the lack of a libm (it is part of the
  implicitly linked in libroot)

Applied-by: Rolf Eike Beer <eike@sf-mail.de>