Remove some forcing of compiler flags

CMakeLists.txt

@@ -48,12 +48,6 @@ option(PHMAP_BUILD_EXAMPLES "Whether or not to build the examples" OFF)
 
 if(MSVC)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /bigobj")
-else()
-    if(CXXFLAGS)
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${CXXFLAGS}")
-    else()
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -O3 -march=native")
-    endif()
 endif()
 
 if (PHMAP_BUILD_TESTS)