CMake config sets HAVE_BMI2 on MSVC even when SNAPPY_REQUIRE_AVX is 0

Experiment: https://godbolt.org/z/H7jymg

clang and gcc do not compile `_bzhi_u32` without `-mbmi2`. So, on these compilers, setting `SNAPPY_REQUIRE_AVX2=0` should result in `HAVE_BMI2` being set to 0, as the compilation test should fail.

MSVC does compile `_bzhi_u32` without `/arch:AVX2`. [The docs](https://docs.microsoft.com/en-us/cpp/build/reference/arch-x64?view=vs-2019) state that the default for 64-bit compilers is SSE2. It seems like the docs are referring to the compiler's instruction selection and that, as you mentioned, MSVC will happily assemble anything you ask it to.

This issue is open while we figure out how to handle the MSVC situation.