[CMake] Fix issues building runtimes

This resolves two issues:
(1) LIBCXX_HEADER_DIR is a very misleadingly named variable because it shouldn't be set to the header directory, instead it needs to be the root binary dir.
(2) If you build runtimes without libcxx, we can't depend on the libcxx header target, so we should instaed refer to it by the variable name which will be unset if libcxx isn't present.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@361646 91177308-0d34-0410-b5e6-96231b3b80d8

runtimes/CMakeLists.txt

@@ -210,7 +210,9 @@ if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
 else() # if this is included from LLVM's CMake
   include(LLVMExternalProjectUtils)
   if (LLVM_EXTERNAL_LIBCXX_SOURCE_DIR AND "libcxx" IN_LIST LLVM_ENABLE_RUNTIMES)
-    set(LIBCXX_HEADER_DIR ${LLVM_INCLUDE_DIR}/c++/v1/)
+    # This looks wrong, but libcxx's build actually wants the header dir to be
+    # the root build dir, not the include directory.
+    set(LIBCXX_HEADER_DIR ${LLVM_BINARY_DIR})
     set(CXX_HEADER_TARGET runtime-libcxx-headers)
     add_subdirectory(${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR}/include ${CXX_HEADER_TARGET})
   endif()
@@ -363,7 +365,7 @@ else() # if this is included from LLVM's CMake
 
     llvm_ExternalProject_Add(runtimes
                              ${CMAKE_CURRENT_SOURCE_DIR}
-                             DEPENDS ${ARG_DEPENDS} runtime-libcxx-headers
+                             DEPENDS ${ARG_DEPENDS} ${CXX_HEADER_TARGET}
                              # Builtins were built separately above
                              CMAKE_ARGS -DCOMPILER_RT_BUILD_BUILTINS=Off
                                         -DLLVM_INCLUDE_TESTS=${LLVM_INCLUDE_TESTS}
@@ -454,7 +456,7 @@ else() # if this is included from LLVM's CMake
 
     llvm_ExternalProject_Add(runtimes-${name}
                              ${CMAKE_CURRENT_SOURCE_DIR}
-                             DEPENDS ${${name}_deps} runtime-libcxx-headers
+                             DEPENDS ${${name}_deps} ${CXX_HEADER_TARGET}
                              # Builtins were built separately above
                              CMAKE_ARGS -DCOMPILER_RT_BUILD_BUILTINS=Off
                                         -DLLVM_INCLUDE_TESTS=${LLVM_INCLUDE_TESTS}