Remove arch specific compiler flags.

While we are not really building anything that is distributed,
these compiler flags don't exist on all architectures.

On PPC this dies with.
  c++: error: unrecognized command line option '-march=native'; did you mean '-mcpu=native'?

CMakeLists.txt

@@ -50,7 +50,7 @@ if(CCACHE_FOUND)
 endif(CCACHE_FOUND)
 
 if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -march=native -Wall -Wextra -Werror")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Werror")
     if (RAPIDJSON_BUILD_CXX11)
         if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS "4.7.0")
             set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x")
@@ -73,7 +73,7 @@ if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
         endif()
     endif()
 elseif (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -march=native -Wall -Wextra -Werror -Wno-missing-field-initializers")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Werror -Wno-missing-field-initializers")
     if (RAPIDJSON_BUILD_CXX11)
         set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
     endif()