Index: Makefile.config.in
===================================================================
--- Makefile.config.in	(revision 227604)
+++ Makefile.config.in	(working copy)
@@ -58,6 +58,10 @@
 PROJ_SRC_ROOT   := $(LLVM_SRC_ROOT)
 PROJ_SRC_DIR    := $(LLVM_SRC_ROOT)$(patsubst $(PROJ_OBJ_ROOT)%,%,$(PROJ_OBJ_DIR))

+ifeq ($(LLVM_SRC_ROOT), $(LLVM_OBJ_ROOT))
+$(error In-source builds are discouraged. 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))
Index: autoconf/configure.ac
===================================================================
--- autoconf/configure.ac	(revision 227604)
+++ autoconf/configure.ac	(working copy)
@@ -73,6 +73,11 @@
   fi
 fi

+dnl Quit if it is an in-source build
+if test ${srcdir} == "." ; then
+  AC_MSG_ERROR([In-source builds are discouraged. Configure from a separate build directory.])
+fi
+
 dnl Default to empty (i.e. assigning the null string to) CFLAGS and CXXFLAGS,
 dnl instead of the autoconf default (for example, '-g -O2' for CC=gcc).
 : ${CFLAGS=}