message: In CMakeLists.txt, when building with PETSc, call enable_language(C),
  which seems to be required by find_package(PETSc REQUIRED).
diff: @@ -53,11 +53,12 @@ if (${PROJECT_SOURCE_DIR} STREQUAL ${PROJECT_BINARY_DIR})
  "MFEM does not support in-source CMake builds at this time.")
  endif (${PROJECT_SOURCE_DIR} STREQUAL ${PROJECT_BINARY_DIR})
  -if (CMAKE_VERSION VERSION_LESS 3.2 OR MFEM_USE_SIDRE)
  +if (CMAKE_VERSION VERSION_LESS 3.2 OR MFEM_USE_SIDRE OR MFEM_USE_PETSC)
  # This seems to be needed by:
  # * find_package(BLAS REQUIRED) and
  # * find_package(HDF5 REQUIRED) needed, in turn, by:
  # - find_package(ATK REQUIRED)
  + # * find_package(PETSc REQUIRED)
  enable_language(C)
  endif()
  