Implement support for external Zofu

[barracuda156]

CMakeLists have the following: https://github.com/josephalevin/fson/blob/16731d9b510b94a4e074a1fa7e35706f6045a85f/CMakeLists.txt#L133 In fact, however, zofu does have CMake implementation. It is also desirable to add an option for using external zofu, so that it does not need to built twice. (Moreover, on macOS internal build of zofu just fails, at least on my system. External works perfectly and passes all tests.)

[acroucher]

Yes, zofu does have a CMake implementation now, so that could probably be changed.

@kprussing, you did the CMake stuff for FSON, would you be interesting in looking into this?

[barracuda156]

Besides, with meson everything worked fine with external zofu as is. Only CMake needs an option or a fix for it to work automatically.

[kprussing]

I'm not 100% sure what is going on with the built version of ZOFU. I just tried a handful of times on macOS and had no trouble building. I'm using gfortran 12.2.0 from MacPorts.