message: Fix arch flags for performance miniapp with gcc
diff: @@ -27,6 +27,7 @@ if (${CMAKE_CXX_COMPILER_ID} MATCHES "Clang")
  "-ffp-contract=fast")
  elseif (${CMAKE_CXX_COMPILER_ID} MATCHES "GNU")
  list(APPEND PERFORMANCE_CXX_OPTIONS
  + ${MFEM_PERF_CXX_ARCH_FLAGS}
  "-Wall")
  if (NOT MFEM_USE_CUDA)
  list(APPEND PERFORMANCE_CXX_OPTIONS "-pedantic")
  