build: Do not use help2man if compiled binaries can't be run.
When cross-compiling, the compiled binaries cannot be run on the build machine, unless one defines a helper (e.g. qemu) in the meson configuration, which is most of the time undesirable.
If help2man is installed on the build machine, then the build would fail because of this. Disable it if the build machine cannot run the compiled binaries.