message: Add PATCH_COMMAND to eigen.cmake
diff: # build_file = "eigen.BUILD",
  #)
  +option(eigen_PATCH_FILE "Patch file to apply to eigen" OFF)
  +set(eigen_PATCH_COMMAND "")
  +if(eigen_PATCH_FILE)
  + set(eigen_PATCH_COMMAND PATCH_COMMAND patch -p0 -i "${eigen_PATCH_FILE}")
  +endif(eigen_PATCH_FILE)
  +
  include (ExternalProject)
  # We parse the current Eigen version and archive hash from the bazel configuration
  @@ -45,6 +51,7 @@ ExternalProject_Add(eigen
  URL ${eigen_URL}
  DOWNLOAD_DIR "${DOWNLOAD_LOCATION}"
  INSTALL_DIR "${eigen_INSTALL}"
  + ${eigen_PATCH_COMMAND}
  CMAKE_CACHE_ARGS
  -DCMAKE_BUILD_TYPE:STRING=Release
  -DCMAKE_VERBOSE_MAKEFILE:BOOL=OFF
  