CMake: support AVX2 in MinGW

Summary:
yuslepukhin

See individual commits.
Closes https://github.com/facebook/rocksdb/pull/2051

Differential Revision: D4819697

Pulled By: siying

fbshipit-source-id: 75c1de4

CMakeLists.txt

@@ -42,11 +42,6 @@ endif()
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake/modules/")
 
 if(WIN32)
-  if (DEFINED AVX2)
-    set(USE_AVX2 ${AVX2})
-  else ()
-    set(USE_AVX2 1)
-  endif ()
   include(${CMAKE_CURRENT_SOURCE_DIR}/thirdparty.inc)
 else()
   option(WITH_JEMALLOC "build with JeMalloc" OFF)
@@ -110,18 +105,24 @@ if (NOT WIN32)
   string(STRIP "${ROCKSDB_VERSION_MAJOR}" ROCKSDB_VERSION_MAJOR)
 endif()
 
+if(WIN32)
+  option(WITH_AVX2 "build with AVX2" ON)
+  if(WITH_AVX2)
+    if(MSVC)
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /arch:AVX2")
+    else()
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -mavx2")
+    endif()
+  endif()
+endif()
+
 set(BUILD_VERSION_CC ${CMAKE_BINARY_DIR}/build_version.cc)
 configure_file(util/build_version.cc.in ${BUILD_VERSION_CC} @ONLY)
 add_library(build_version OBJECT ${BUILD_VERSION_CC})
 target_include_directories(build_version PRIVATE
   ${CMAKE_CURRENT_SOURCE_DIR}/util)
 if(MSVC)
-  if (${USE_AVX2} EQUAL 1)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /Zi /nologo  /arch:AVX2 /EHsc /GS /Gd /GR /GF /fp:precise /Zc:wchar_t /Zc:forScope /errorReport:queue")
-  else ()
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /Zi /nologo /EHsc /GS /Gd /GR /GF /fp:precise /Zc:wchar_t /Zc:forScope /errorReport:queue")
-  endif ()
-
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /FC /d2Zi+ /W3 /wd4127 /wd4800 /wd4996 /wd4351")
 else()
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -W -Wextra -Wall")