Index: cmake/caches/Linux.cmake
===================================================================
--- /dev/null
+++ cmake/caches/Linux.cmake
@@ -0,0 +1,60 @@
+# This file is primarily for cross compiling clang+llvm, et al, for
+# Linux on Darwin, and can be invoked like this:
+#
+#  cmake -GNinja -DBOOTSTRAP_CMAKE_SYSROOT=<path> [OPTIONS] \
+#    -DBOOTSTRAP_CMAKE_TOOLCHAIN_FILE=<llvm root>/cmake/platforms/Linux.cmake \
+#    -C <clang root>/cmake/caches/Linux.cmake ../llvm
+
+if(NOT DEFINED BOOTSTRAP_CMAKE_SYSROOT)
+  message(SEND_ERROR "Missing required argument -DBOOTSTRAP_CMAKE_SYSROOT=<sysroot path>.")
+endif()
+if(NOT DEFINED BOOTSTRAP_CMAKE_TOOLCHAIN_FILE)
+  message(SEND_ERROR "Missing required argument -DBOOTSTRAP_CMAKE_TOOLCHAIN_FILE=<file>.")
+endif()
+
+set(LLVM_TARGETS_TO_BUILD Native CACHE STRING "")
+set(CMAKE_BUILD_TYPE RELEASE CACHE STRING "")
+set(LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
+
+set(CLANG_ENABLE_BOOTSTRAP ON CACHE BOOL "")
+
+if(NOT DEFINED BOOTSTRAP_LLVM_DEFAULT_TARGET_TRIPLE)
+  set(BOOTSTRAP_LLVM_DEFAULT_TARGET_TRIPLE "x86_64-unknown-linux-gnu")
+endif()
+
+# Since LLVM_ENABLE_PROJECTS gets passed automatically to the next
+# stage, use another variable to pass the desired projects to stage2.
+if(DEFINED BOOTSTRAP_LLVM_ENABLE_PROJECTS)
+  set(BOOTSTRAP_LLVM_ENABLE_PROJECTS_OVERRIDE ${BOOTSTRAP_LLVM_ENABLE_PROJECTS} CACHE STRING "" FORCE)
+  unset(BOOTSTRAP_LLVM_ENABLE_PROJECTS CACHE)
+else()
+  set(BOOTSTRAP_LLVM_ENABLE_PROJECTS_OVERRIDE "clang;libcxx;libcxxabi;libunwind" CACHE STRING "" FORCE)
+endif()
+
+if(CMAKE_HOST_APPLE)
+  # Make sure at least clang and lld are included.
+  list(APPEND LLVM_ENABLE_PROJECTS clang lld)
+  set(LLVM_ENABLE_PROJECTS ${LLVM_ENABLE_PROJECTS} CACHE STRING "")
+
+  # Passing -fuse-ld=lld is hard for cmake to handle correctly, so
+  # make lld the default linker.
+  set(CLANG_DEFAULT_LINKER lld CACHE STRING "" FORCE)
+  set(BOOTSTRAP_LLVM_ENABLE_LLD ON CACHE BOOL "")
+
+  set(BOOTSTRAP_CMAKE_AR ${CMAKE_BINARY_DIR}/bin/llvm-ar CACHE STRING "")
+  set(BOOTSTRAP_CMAKE_RANLIB ${CMAKE_BINARY_DIR}/bin/llvm-ranlib CACHE STRING "")
+  set(BOOTSTRAP_CLANG_TABLEGEN ${CMAKE_BINARY_DIR}/bin/clang-tblgen CACHE STRING "")
+  set(BOOTSTRAP_LLVM_TABLEGEN ${CMAKE_BINARY_DIR}/bin/llvm-tblgen CACHE STRING "")
+  set(BOOTSTRAP_LLVM_CONFIG_EXE ${CMAKE_BINARY_DIR}/bin/llvm-config CACHE STRING "")
+endif()
+
+# Since we just want to build a bootstrap compiler, turn off as much
+# as possible.
+set(LLVM_INCLUDE_DOCS OFF CACHE BOOL "")
+set(LLVM_INCLUDE_EXAMPLES OFF CACHE BOOL "")
+set(LLVM_INCLUDE_RUNTIMES OFF CACHE BOOL "")
+set(LLVM_INCLUDE_TESTS OFF CACHE BOOL "")
+set(LLVM_INCLUDE_UTILS OFF CACHE BOOL "")
+set(CLANG_BUILD_TOOLS OFF CACHE BOOL "")
+set(CLANG_ENABLE_ARCMT OFF CACHE BOOL "")
+set(CLANG_ENABLE_STATIC_ANALYZER OFF CACHE BOOL "")