+# The macro below is expanded when 'realpath' is not built-in.
+# Built-in 'realpath' is available on GNU Make 3.81.
+realpath = $(shell cd $(1); $(PWD))
+
+PROJ_OBJ_DIR := $(call realpath, .)
+PROJ_OBJ_ROOT := $(call realpath, $(PROJ_OBJ_DIR)/$(LEVEL))
+
+CLANG_SRC_ROOT := @CLANG_SRC_ROOT@
+
+ifeq ($(PROJECT_NAME),$(LLVMPackageName))
+LLVM_SRC_ROOT := $(call realpath, @abs_top_srcdir@)
+LLVM_OBJ_ROOT := $(call realpath, @abs_top_builddir@)
+PROJ_SRC_ROOT := $(LLVM_SRC_ROOT)
+PROJ_SRC_DIR := $(LLVM_SRC_ROOT)$(patsubst $(PROJ_OBJ_ROOT)%,%,$(PROJ_OBJ_DIR))
+
+# FIXME: This is temporary during the grace period where in-source builds are
+# deprecated. Convert to a hard error when that period is up.
+#
+# See: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150323/268067.html
+ifeq ($(LLVM_SRC_ROOT), $(LLVM_OBJ_ROOT))
+ $(warning ######################################################################################)
+ $(warning # #)
+ $(warning # WARNING #)
+ $(warning # #)
+ $(warning # In-source builds are deprecated. #)
+ $(warning # #)
+ $(warning # Please configure from a separate build directory! #)
+ $(warning # #)
+ $(warning ######################################################################################)
+endif
+
+ifneq ($(CLANG_SRC_ROOT),)
+ CLANG_SRC_ROOT:= $(call realpath, $(CLANG_SRC_ROOT))
+ PROJ_SRC_DIR := $(patsubst $(LLVM_SRC_ROOT)/tools/clang%,$(CLANG_SRC_ROOT)%,$(PROJ_SRC_DIR))
+endif