Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -732,6 +732,8 @@
   add_subdirectory(tools)
 endif()
 
+add_subdirectory(runtimes)
+
 if( LLVM_INCLUDE_EXAMPLES )
   add_subdirectory(examples)
 endif()
@@ -742,7 +744,8 @@
     llvm_ExternalProject_Add(test-suite ${LLVM_MAIN_SRC_DIR}/projects/test-suite
       USE_TOOLCHAIN
       EXCLUDE_FROM_ALL
-      NO_INSTALL)
+      NO_INSTALL
+      ALWAYS_CLEAN)
   endif()
   add_subdirectory(test)
   add_subdirectory(unittests)
Index: cmake/modules/LLVMExternalProjectUtils.cmake
===================================================================
--- cmake/modules/LLVMExternalProjectUtils.cmake
+++ cmake/modules/LLVMExternalProjectUtils.cmake
@@ -19,6 +19,8 @@
 #     Exclude this project from the all target
 #   NO_INSTALL
 #     Don't generate install targets for this project
+#   ALWAYS_CLEAN
+#     Always clean the sub-project before building
 #   CMAKE_ARGS arguments...
 #     Optional cmake arguments to pass when configuring the project
 #   TOOLCHAIN_TOOLS targets...
@@ -27,11 +29,15 @@
 #     Targets that this project depends on
 #   EXTRA_TARGETS targets...
 #     Extra targets in the subproject to generate targets for
+#   PASSTHROUGH_PREFIXES prefix...
+#     Extra variable prefixes (name is always included) to pass down
 #   )
 function(llvm_ExternalProject_Add name source_dir)
-  cmake_parse_arguments(ARG "USE_TOOLCHAIN;EXCLUDE_FROM_ALL;NO_INSTALL"
+  cmake_parse_arguments(ARG
+    "USE_TOOLCHAIN;EXCLUDE_FROM_ALL;NO_INSTALL;ALWAYS_CLEAN"
     "SOURCE_DIR"
-    "CMAKE_ARGS;TOOLCHAIN_TOOLS;RUNTIME_LIBRARIES;DEPENDS;EXTRA_TARGETS" ${ARGN})
+    "CMAKE_ARGS;TOOLCHAIN_TOOLS;RUNTIME_LIBRARIES;DEPENDS;EXTRA_TARGETS;PASSTHROUGH_PREFIXES"
+    ${ARGN})
   canonicalize_tool_name(${name} nameCanon)
   if(NOT ARG_TOOLCHAIN_TOOLS)
     set(ARG_TOOLCHAIN_TOOLS clang lld)
@@ -52,6 +58,10 @@
     endif()
   endforeach()
 
+  if(ARG_ALWAYS_CLEAN)
+    set(always_clean clean)
+  endif()
+
   list(FIND TOOLCHAIN_TOOLS clang FOUND_CLANG)
   if(FOUND_CLANG GREATER -1)
     set(CLANG_IN_TOOLCHAIN On)
@@ -71,15 +81,18 @@
     USES_TERMINAL
     )
 
-  # Find all variables that start with COMPILER_RT and populate a variable with
-  # them.
+  # Find all variables that start with a prefix and propagate them through
   get_cmake_property(variableNames VARIABLES)
-  foreach(variableName ${variableNames})
-    if(variableName MATCHES "^${nameCanon}")
-      string(REPLACE ";" "\;" value "${${variableName}}")
-      list(APPEND PASSTHROUGH_VARIABLES
-        -D${variableName}=${value})
-    endif()
+
+  list(APPEND ARG_PASSTHROUGH_PREFIXES ${nameCanon})
+  foreach(prefix ${ARG_PASSTHROUGH_PREFIXES})
+    foreach(variableName ${variableNames})
+      if(variableName MATCHES "^${prefix}")
+        string(REPLACE ";" "\;" value "${${variableName}}")
+        list(APPEND PASSTHROUGH_VARIABLES
+          -D${variableName}=${value})
+      endif()
+    endforeach()
   endforeach()
 
   if(ARG_USE_TOOLCHAIN)
@@ -117,6 +130,12 @@
     CMAKE_ARGS ${${nameCanon}_CMAKE_ARGS}
                ${compiler_args}
                -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
+               -DLLVM_BINARY_DIR=${PROJECT_BINARY_DIR}
+               -DLLVM_CONFIG_PATH=$<TARGET_FILE:llvm-config>
+               -DLLVM_ENABLE_WERROR=${LLVM_ENABLE_WERROR}
+               -DPACKAGE_VERSION=${PACKAGE_VERSION}
+               -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
+               -DCMAKE_MAKE_PROGRAM=${CMAKE_MAKE_PROGRAM}
                ${ARG_CMAKE_ARGS}
                ${PASSTHROUGH_VARIABLES}
     INSTALL_COMMAND ""
@@ -138,6 +157,7 @@
     DEPENDEES configure
     ${force_deps}
     WORKING_DIRECTORY ${BINARY_DIR}
+    EXCLUDE_FROM_MAIN 1
     USES_TERMINAL 1
     )
   ExternalProject_Add_StepTargets(${name} clean)
Index: runtimes/CMakeLists.txt
===================================================================
--- /dev/null
+++ runtimes/CMakeLists.txt
@@ -0,0 +1,67 @@
+# Discover the projects that use CMake in the subdirectories.
+# Note that explicit cmake invocation is required every time a new project is
+# added or removed.
+
+file(GLOB entries *)
+foreach(entry ${entries})
+  if(IS_DIRECTORY ${entry} AND EXISTS ${entry}/CMakeLists.txt)
+    list(APPEND runtimes ${entry})
+  endif()
+endforeach()
+
+if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
+
+  cmake_minimum_required(VERSION 3.4.3)
+  
+  list(INSERT CMAKE_MODULE_PATH 0
+    "${CMAKE_CURRENT_SOURCE_DIR}/../cmake"
+    "${CMAKE_CURRENT_SOURCE_DIR}/../cmake/Modules"
+    "${LLVM_BINARY_DIR}/lib/cmake/llvm"
+  )
+
+  include(LLVMConfig)
+
+  set(LLVM_LIBRARY_OUTPUT_INTDIR ${LLVM_LIBRARY_DIR})
+  set(LLVM_RUNTIME_OUTPUT_INTDIR ${LLVM_BINARY_DIR})
+
+  foreach(entry ${runtimes})
+    get_filename_component(projName ${entry} NAME)
+
+    # TODO: Clean this up as part of an interface standardization
+    string(REPLACE "-" "_" canon_name ${projName})
+    string(TOUPPER ${canon_name} canon_name)
+    # The subdirectories need to treat this as standalone builds
+    set(${canon_name}_STANDALONE_BUILD On)
+    set(${canon_name}_BUILT_STANDALONE On)
+
+    add_subdirectory(${projName})
+  endforeach()
+
+else()
+  include(LLVMExternalProjectUtils)
+
+  if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/compiler-rt)
+    llvm_ExternalProject_Add(builtins
+                             ${CMAKE_CURRENT_SOURCE_DIR}/compiler-rt/lib/builtins
+                             PASSTHROUGH_PREFIXES COMPILER_RT
+                             USE_TOOLCHAIN)
+    set(deps builtins)
+  endif()
+
+  foreach(entry ${runtimes})
+    get_filename_component(projName ${entry} NAME)
+    string(REPLACE "-" "_" canon_name ${projName})
+    string(TOUPPER ${canon_name} canon_name)
+    list(APPEND prefixes ${canon_name})
+  endforeach()
+
+  if(runtimes)
+    llvm_ExternalProject_Add(runtimes
+                             ${CMAKE_CURRENT_SOURCE_DIR}
+                             DEPENDS ${deps} llvm-config
+                             # Builtins were built separately above
+                             CMAKE_ARGS -DCOMPILER_RT_BUILD_BUILTINS=Off
+                             PASSTHROUGH_PREFIXES ${prefixes}
+                             USE_TOOLCHAIN)
+  endif()
+endif()