build: Start sketching code to allow grafting libc++ build into LLVM/Clang build if

libc++ is checked out into llvm/projects.
 - WIP, not on by default yet.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@113924 91177308-0d34-0410-b5e6-96231b3b80d8

runtime/libcxx/Makefile

+##===- clang/runtime/libcxx/Makefile -----------------------*- Makefile -*-===##
+#
+#                     The LLVM Compiler Infrastructure
+#
+# This file is distributed under the University of Illinois Open Source
+# License. See LICENSE.TXT for details.
+#
+##===----------------------------------------------------------------------===##
+#
+# This file defines support for building the "libc++" C++ standard library as
+# part of a Clang compiler build.
+#
+##===----------------------------------------------------------------------===##
+
+CLANG_LEVEL := ../..
+LEVEL := $(CLANG_LEVEL)/../..
+LIBRARYNAME = c++
+
+LINK_LIBS_IN_SHARED = 1
+SHARED_LIBRARY = 1
+
+# Include LLVM common makefile.
+include $(LEVEL)/Makefile.config
+
+# Expect libcxx to be in llvm/projects/libcxx
+LIBCXX_SRC_ROOT := $(LLVM_SRC_ROOT)/projects/libcxx
+
+# Override the source root to point at the libcxx sources.
+PROJ_MAKEFILE := $(PROJ_SRC_DIR)/Makefile
+PROJ_SRC_DIR := $(LIBCXX_SRC_ROOT)/src
+CPP.Flags := -nostdinc++ -I$(LIBCXX_SRC_ROOT)/include
+CXX.Flags := -std=c++0x
+
+# Include LLVM makefile rules.
+include $(LLVM_SRC_ROOT)/Makefile.rules
+
+# Force building with the just built Clang.
+#
+# FIXME: Do we really want to do this? It is uber slow.
+# CXX := $(ToolDir)/clang
+
+ifeq ($(HOST_OS),Darwin)
+    LLVMLibsOptions += -Wl,-compatibility_version,1
+
+    # Don't link with default libraries.
+    LLVMLibsOptions += -nodefaultlibs -lSystem
+
+    # Reexport libc++abi.
+    LLVMLibsOptions += -Wl,-reexport_library,/usr/lib/libc++abi.dylib
+
+    # Set dylib internal version number to submission number.
+    ifdef LLVM_SUBMIT_VERSION
+        LLVMLibsOptions += -Wl,-current_version \
+                           -Wl,$(LLVM_SUBMIT_VERSION).$(LLVM_SUBMIT_SUBVERSION)
+    endif
+
+    # Mac OS X 10.4 and earlier tools do not allow a second -install_name on command line
+    DARWIN_VERS := $(shell echo $(TARGET_TRIPLE) | sed 's/.*darwin\([0-9]*\).*/\1/')
+    ifneq ($(DARWIN_VERS),8)
+       LLVMLibsOptions += -Wl,-install_name \
+                          -Wl,"@executable_path/../lib/lib$(LIBRARYNAME)$(SHLIBEXT)"
+    endif
+endif