In adding cross-build description patch, I added the method to build doc on other platforms. However, some programs installed in /usr/bin/ may need to be added to /usr/local/bin/ because some package managers such as dnf and apt instsall in /usr/bin/. If this patch makes Makefile complicated, there is another solution to specify only the command. (for example, PYTHON_CMD=python3 instead of PYTHON_CMD=/usr/bin/local/python3)
Works fine on macOS and FreeBSD. It does not may on Linux, but it is the problem of distro. (see https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=255804)
I object to the platform-dependent changes to the Makefiles. It sounds unnecessary and error-prone to me. For example, different flavors of GNU/Linux could have different executable locations, and we can't and shouldn't attempt to support all of them.