configure/CMake + SIMD/pthreads

Talking with @surma it sounds like configure and CMake may not work with new features like SIMD and pthreads. That is, if the developer wants to use SIMD, and the configure check runs on a node version which is too old to have it, the check would fail.

The question is, do configure scripts do such checks, and whether we need to do something for it. If we do, options might include making it easier to use newer node or v8, and/or giving clear errors for how to fix this or work around it, and docs.

If relevant, we may want to add configure tests to our test suite for this. I'm pretty sure the existing configure tests do not use SIMD or pthreads.

I'm not sure exactly which flags users would need for SIMD, and which ones a configure script would check. Presumably only the SSE emulation stuff would be relevant here? For pthreads, the normal `-pthread` flag would be relevant I assume.

cc @tlively