Make SIMD capability variables thread-local ... ... on platforms that support TLS, which should include all currently-supported platforms (https://libjpeg-turbo.org/Documentation/OfficialBinaries) Addresses a concern raised in #87 Although it is still my opinion that the data race in init_simd() was innocuous, we can now fix it for free thanks to ae87a958613b69628b92088b313ded0d4f59a716, so why not?