How to create a cross-compiler on LINUX (or any other supported build platform)
for haiku (information is copied from haiku's README):

Please chdir into haiku's base directory (the one where Jamrules and README live).

If you want to build the default (legacy, 2.95.3) gcc, do this:

  $ ./configure --build-cross-tools ../buildtools

One of the last output lines should tell you that the tools have been built
successfully.

If you're not interested in binary compatibility (or want to build for the PowerPC architecture), you can build gcc5 instead by doing this:

  $ ./configure --build-cross-tools <arch> ../buildtools

Replace "<arch>" with either "x86" or "ppc", depending on which of the two
architectures you want to build for.