Add -mavx2 for TARGET=HASWELL if compiler supports and requires it

cmake/system.cmake

@@ -45,6 +45,12 @@ if (DEFINED TARGET)
 if (${TARGET} STREQUAL "SKYLAKEX" AND NOT NO_AVX512)
   set (KERNEL_DEFINITIONS "${KERNEL_DEFINITIONS} -march=skylake-avx512")
 endif()
+if (${TARGET} STREQUAL "HASWELL" AND NOT NO_AVX2)
+   execute_process(COMMAND ${CMAKE_C_COMPILER} -dumpversion OUTPUT_VARIABLE GCC_VERSION)
+   if (${GCC_VERSION} VERSION_GREATER 4.7 OR ${GCC_VERSION} VERSION_EQUAL 4.7)
+     set (KERNEL_DEFINITIONS "${KERNEL_DEFINITIONS} -mavx2")
+   endif()
+endif()
 endif()
 
 if (DEFINED TARGET)