message: Remove unsupported mtune option
diff: @@ -56,7 +56,7 @@ MFEM_PERF_CXXFLAGS_gcc_ppc64 = -mcpu=native -mtune=native\
  $(MFEM_PERF_CXXFLAGS_gcc_common)
  # - XLC extra options:
  -MFEM_PERF_CXXFLAGS_xlc = -mcpu=native -mtune=native
  +MFEM_PERF_CXXFLAGS_xlc = -mcpu=native
  # - Clang extra options:
  MFEM_PERF_CXXFLAGS_clang += -march=native
  