Wrap BMI2 instruction usage in support checks.

A previous version of this was submitted and rolled back due to breakage
-- an attempt to accommodate Visual Studio resulted in compiler errors
on GCC/Clang with -mavx2 but without -mbmi2. This version makes the BMI2
support check more strict, to avoid the errors.

A previous CL introduced _bzhi_u32 (part of Intel's BMI2 instruction
set, released in Haswell) gated by a check for the __BMI2__ preprocessor
macro. This works for Clang and GCC, but does not work on Visual Studio,
and may not work on other compilers.

This CL plumbs the BMI2 support checks through the CMake configuration
used by the open source build. It also replaces the <x86intrin.h>
header, which does not exist on Visual Studio, with the more scoped
headers <tmmintrin.h> (for SSSE3) and <immintrin.h> (for BMI2/AVX2).
Asides from fixing the open source build, the more scoped headers make
it slightly less likely that newer intrinsics will creep in without
proper gating.

.appveyor.yml

@@ -30,7 +30,7 @@ build_script:
   - if "%platform%"=="x64" set CMAKE_GENERATOR=%CMAKE_GENERATOR% Win64
   - cmake --version
   - cmake .. -G "%CMAKE_GENERATOR%"
-      -DCMAKE_CONFIGURATION_TYPES="%CONFIGURATION%" -DSNAPPY_REQUIRE_AVX=ON
+      -DCMAKE_CONFIGURATION_TYPES="%CONFIGURATION%" -DSNAPPY_REQUIRE_AVX2=ON
   - cmake --build . --config %CONFIGURATION%
   - cd ..