Codechange: [OSX] Prevent the compiler from using SSE4 instructions unless we want to.

This improves compatibility on older systems. We don't need to disable anything
older, as there are no SSE2-only Macs.

cmake/CompileFlags.cmake

@@ -127,6 +127,18 @@ macro(compile_flags)
                 "$<$<BOOL:${LIFETIME_DSE_FOUND}>:-flifetime-dse=1>"
             )
         endif()
+
+        if(CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang")
+            include(CheckCXXCompilerFlag)
+            check_cxx_compiler_flag("-mno-sse4" NO_SSE4_FOUND)
+
+            if(NO_SSE4_FOUND)
+                add_compile_options(
+                    # Don't use SSE4 for general sources to increase compatibility.
+                    -mno-sse4
+                )
+            endif()
+        endif()
     elseif(CMAKE_CXX_COMPILER_ID STREQUAL "Intel")
         add_compile_options(
             -Wall