add ffc to our benchmarks.

[lemire]

(Requires LLVM/clang to test ffc.)

$ cmake -B build
$ cmake --build build
$ ./build/benchmarks/benchmark -f data/canada.txt 
# read 111126 lines 
ASCII volume = 1.93374 MB 
Permission denied, xnu/kpc requires root privileges.
netlib                                  :   605.90 MB/s (+/- 6.4 %)    34.82 Mfloat/s  
strtod                                  :  1144.20 MB/s (+/- 9.1 %)    65.75 Mfloat/s  
abseil                                  :  1232.27 MB/s (+/- 10.3 %)    70.81 Mfloat/s  
fastfloat                               :  1617.97 MB/s (+/- 7.3 %)    92.98 Mfloat/s  
ffc                                     :  1775.44 MB/s (+/- 10.3 %)   102.03 Mfloat/s  
UTF-16 volume = 3.86749 MB 
fastfloat                               :  3188.91 MB/s (+/- 11.1 %)    91.63 Mfloat/s  

@kolemannix

[kolemannix]

I see the benchmarks failed to compile due to the VLA hint (size_t len, const char input[len]); I might just remove it for improved cpp compatibility.

[lemire]

@kolemannix It works with LLVM/clang.

For now, I will just require LLVM/clang.