message: Mention that MFEM_USE_LEGACY_OPENMP is deprecated in INSTALL.
diff: @@ -278,6 +278,7 @@ MFEM_THREAD_SAFE = YES/NO
  MFEM_USE_LEGACY_OPENMP = YES/NO
  Enable (basic) experimental OpenMP support. Requires MFEM_THREAD_SAFE.
  + This option is deprecated.
  MFEM_USE_OPENMP = YES/NO
  Enable the OpenMP backend.
  