Only the simdjson library should optionally depend on threads,
the executables that link to simdjson will get the dependency
indirectly.
* add option for controlling threads (default is on)
* add CI testing with threading on/off for msvc, gcc and clang
* fix an unrelated copy paste comment error in the cirlce ci build conf
* Hiding the runtime dispatch pointer in a source file so it is not an exported symbol
* Disabling hard failure on style check.
* Fixes https://github.com/lemire/simdjson/issues/250
John Keiser
Paul Dreik
Daniel Lemire