The header guard in the file src/core/errors.h is currently set to
_ERRORS_H but this conflicts with system header files on Haiku OS. Perhaps it could be changed to
_SCERRORS_H or some other value.
SpeedCrunch works great on Haiku(screenshot). I may submit a small pull request later with minor additions to the CMakeLists.txt file(mainly to change the install location since Haiku prefers /apps for GUI apps instead of /bin).