Check and use only available FFTW libraries

[devmotion]

I think it would be good to check if FFTW_jll and MKL_jll are actually available before trying to load them, as suggested in https://github.com/JuliaMath/FFTW.jl/issues/274#issuecomment-1618774373. I ran into this issue with a project that specified provider = "mkl" in a LocalPreferences.toml - precompilation will error on my MacBook since MKL is not available for Apple silicon. With this PR, FFTW will use FFTW_jll instead (if it's not available either an error is thrown) but present the user with the error message:

┌ FFTW [7a1cc6ca-52ef-59f5-83cd-3a7055c09341]
│  ┌ Error: Invalid provider setting "mkl"; valid settings include ["fftw"]
│  └ @ FFTW ~/.julia/dev/FFTW/src/providers.jl:23
└

Fixes https://github.com/JuliaMath/FFTW.jl/issues/274.

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (ef8fc5b) 73.08% compared to head (9db530c) 73.15%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
+ Coverage   73.08%   73.15%   +0.07%     
==========================================
  Files           5        5              
  Lines         535      529       -6     
==========================================
- Hits          391      387       -4     
+ Misses        144      142       -2     

Files	Coverage Δ
src/providers.jl	66.66% <60.00%> (+7.40%)	:arrow_up:

... and 2 files with indirect coverage changes

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