Now, compiling in the ghc folder starts correctly and I get errors about missing itimer (can I safely disable that or should I add an emulator for it?) and mmap. Will the libraries be compiled after that?
Those errors are coming from the runtime system in ghc/rts. Both functions are used in fairly important spots in the runtime system: mmap is used to allocate memory for the Haskell program - the memory has to be allocated to multiples of 1MB; you'll need to find some way to do that on BeOS. The itimer is used for supporting multiple threads in Haskell code; you might get away with disabling that until you try to run multithreaded code, but something definitely needs to be done about this to get a proper BeOS port. Once you're through that, it'll compile the compiler proper in ghc/compiler, is quite likely to just work (unless there are problems with the build system). Then that compiler will be used to compile the libraries, and you'll probably encounter some more problems because of functions missing from BeOS there.
I saw comments about BeOS in the code. Did someone try to port it already?
No, I don't know of any attempts. I didn't even find any reference to BeOS in the sources outside config.guess/config.sub and of course ghc/rts/gmp, which is just an old version of the GNU MP library. GNU MP has probably(hopefully) been ported to BeOS. Grüße, Wolfgang