simdjson: Add clear error when AVX2 is not supported on the current arch

I tried running the benchmarks on my machine but got an error when running the parser. I cloned the repo, ran cmake ., make, then make test to get this result. See screenfetch at bottom for specs of my machine.

Test project /Users/speleo/Downloads/simdjson
    Start 1: jsoncheck
1/1 Test #1: jsoncheck ........................***Exception: Illegal  0.01 sec

0% tests passed, 1 tests failed out of 1

Total Test time (real) =   0.01 sec

The following tests FAILED:
	  1 - jsoncheck (ILLEGAL)
Errors while running CTest
make: *** [test] Error 8

                    'c.          speleo@leaf.local
                 ,xNMM.          -----------------
               .OMMMMo           OS: macOS 10.14 18A391 x86_64
               OMMM0,            Host: MacBookPro10,1
     .;loddo:' loolloddol;.      Kernel: 18.0.0
   cKMMMMMMMMMMNWMMMMMMMMMM0:    Uptime: 7 days, 12 hours, 19 mins
 .KMMMMMMMMMMMMMMMMMMMMMMMWd.    Packages: 273
 XMMMMMMMMMMMMMMMMMMMMMMMX.      Shell: zsh 5.3
;MMMMMMMMMMMMMMMMMMMMMMMM:       Resolution: 1440x900
:MMMMMMMMMMMMMMMMMMMMMMMM:       DE: Aqua
.MMMMMMMMMMMMMMMMMMMMMMMMX.      WM: Kwm
 kMMMMMMMMMMMMMMMMMMMMMMMMWd.    Terminal: iTerm2
 .XMMMMMMMMMMMMMMMMMMMMMMMMMMk   CPU: Intel i7-3635QM (8) @ 2.40GHz
  .XMMMMMMMMMMMMMMMMMMMMMMMMK.   GPU: Intel HD Graphics 4000, NVIDIA GeForce GT 650M
    kMMMMMMMMMMMMMMMMMMMMMMd     Memory: 3678MiB / 16384MiB
     ;KMMMMMMMWXXWMMMMMMMk.
       .cooc,.    .,coo:.