-march=native is not supported by LLVM 13 on M1 Mac (#774)

Author: regehr

CMakeLists.txt

@@ -19,7 +19,7 @@ if (MSVC)
   set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "/O2 /Oi /Zi ${CMAKE_CXX_FLAGS_RELWITHDEBINFO}")
   set(CMAKE_EXE_LINKER_FLAGS         "/LTCG:INCREMENTAL ${CMAKE_EXE_LINKER_FLAGS}")
 else()
-  set(CMAKE_CXX_FLAGS                "-Wall -Werror -march=native -fPIC ${CMAKE_CXX_FLAGS}")
+  set(CMAKE_CXX_FLAGS                "-Wall -Werror -fPIC ${CMAKE_CXX_FLAGS}")
   set(CMAKE_CXX_FLAGS_DEBUG          "-g -Og ${CMAKE_CXX_FLAGS_DEBUG}")
   set(CMAKE_CXX_FLAGS_RELEASE        "-O3 ${CMAKE_CXX_FLAGS_RELEASE}")
   set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS_RELEASE} -g ${CMAKE_CXX_FLAGS_RELWITHDEBINFO}")