[FFTW] Build for experimental platforms

[jeremiahpslewis]

FFTW.jl now requires Julia 1.6. In order to support experimental platforms like Apple Silicon, a new FFTW_jll needs to be built with an updated version number and then FFTW.jl can be updated. This PR seeks to resolve the open point for FFTW in #2763.

[staticfloat]

I'm seeing if I can get an actual new release one more time FFTW/fftw3#248 (comment)

[jeremiahpslewis]

@staticfloat Quick question, the 1:1 mapping of (raw) binary versions to Julia _jll versions is ingenious and simplifies many things except where it makes things infinitely more complicated like with the min 1.6 version / adding experimental platforms transition. Any thoughts about whether this set up feels 'solved' or worth tweaking?

[staticfloat]

Absolutely! Mosè and I have discussed this at length. The critical design error is that we try to shove semantically useful information into the "build number" of the JLL version number (e.g. the X in v1.2.3+X). While this worked just fine while I was prototyping this system out, eventually we realized that the Julia package resolver doesn't allow packages to change attributes about itself if the major, minor and patch versions are the same. 😞

After a lot of back and forth with the Pkg team, (of which I am also a part) we decided that we would eventually decouple JLL version numbers from upstream source numbers, and we would have an automated tool to help keep track of what the "true" version number is. So, as an example, you would have a table somewhere that looks something like:

fftw	FFTW_jll
v3.3.8	v3.3.8
v3.3.9	v3.3.9
v3.3.9	v4.0.0

And then we would have tooling to allow you to do things like ask "what version of the fftw binary do I actually have?", and "Allow me to specify version requirements on the actual library version itself", which would basically translate to the JLL version number ranges.

[staticfloat]

To be clear though, this tooling doesn't exist yet. :)

[jeremiahpslewis]

@staticfloat Nice! Thanks for the clear explanation, makes a lot of sense!

[staticfloat]

The chain of notifications went from me to Valentin to Steven to Matteo, but we finally managed to get a new version of FFTW released!