PROJ_SRC_ROOT := $(LLVM_SRC_ROOT)
PROJ_SRC_DIR := $(LLVM_SRC_ROOT)$(patsubst $(PROJ_OBJ_ROOT)%,%,$(PROJ_OBJ_DIR))
+# See: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150323/268067.html
+ifeq ($(LLVM_SRC_ROOT), $(LLVM_OBJ_ROOT))
+ $(error In-source builds are not allowed. Please configure from a separate build directory!)
+endif
+
ifneq ($(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))