Index: head/Makefile.inc1
===================================================================
--- head/Makefile.inc1
+++ head/Makefile.inc1
@@ -337,6 +337,7 @@
 				COMPILER_TYPE \
 				COMPILER_FEATURES \
 				COMPILER_FREEBSD_VERSION \
+				COMPILER_RESOURCE_DIR \
 				LINKER_VERSION \
 				LINKER_FEATURES \
 				LINKER_TYPE \
Index: head/share/mk/bsd.compiler.mk
===================================================================
--- head/share/mk/bsd.compiler.mk
+++ head/share/mk/bsd.compiler.mk
@@ -143,7 +143,7 @@
 # The value is only used/exported for the same environment that impacts
 # CC and COMPILER_* settings here.
 _exported_vars=	${X_}COMPILER_TYPE ${X_}COMPILER_VERSION \
-		${X_}COMPILER_FREEBSD_VERSION
+		${X_}COMPILER_FREEBSD_VERSION ${X_}COMPILER_RESOURCE_DIR
 ${X_}_cc_hash=	${${cc}}${MACHINE}${PATH}
 ${X_}_cc_hash:=	${${X_}_cc_hash:hash}
 # Only import if none of the vars are set somehow else.
@@ -201,6 +201,10 @@
 .endif
 .endif
 
+.if !defined(${X_}COMPILER_RESOURCE_DIR)
+${X_}COMPILER_RESOURCE_DIR!=	${${cc}:N${CCACHE_BIN}} -print-resource-dir 2>/dev/null || echo unknown
+.endif
+
 ${X_}COMPILER_FEATURES=
 .if (${${X_}COMPILER_TYPE} == "clang" && ${${X_}COMPILER_VERSION} >= 30300) || \
 	(${${X_}COMPILER_TYPE} == "gcc" && ${${X_}COMPILER_VERSION} >= 40800)
@@ -224,6 +228,7 @@
 X_COMPILER_VERSION=	${COMPILER_VERSION}
 X_COMPILER_FREEBSD_VERSION=	${COMPILER_FREEBSD_VERSION}
 X_COMPILER_FEATURES=	${COMPILER_FEATURES}
+X_COMPILER_RESOURCE_DIR=	${COMPILER_RESOURCE_DIR}
 .endif	# ${cc} == "CC" || (${cc} == "XCC" && ${XCC} != ${CC})
 
 # Export the values so sub-makes don't have to look them up again, using the
Index: head/share/mk/bsd.sys.mk
===================================================================
--- head/share/mk/bsd.sys.mk
+++ head/share/mk/bsd.sys.mk
@@ -206,6 +206,21 @@
 # This warning is utter nonsense
 CFLAGS+=	-Wno-format-zero-length
 
+.if ${COMPILER_TYPE} == "clang"
+# The headers provided by clang are incompatible with the FreeBSD headers.
+# If the version of clang is not one that has been patched to omit the
+# incompatible headers, we need to compile with -nobuiltininc and add the
+# resource dir to the end of the search paths. This ensures that headers such as
+# immintrin.h are still found but stddef.h, etc. are picked up from FreeBSD.
+#
+# XXX: This is a hack to support complete external installs of clang while
+# we work to synchronize our decleration guards with those in the clang tree.
+.if ${MK_CLANG_BOOTSTRAP} == "no" && ${COMPILER_RESOURCE_DIR} != "unknown" && \
+    !defined(BOOTSTRAPPING)
+CFLAGS+=-nobuiltininc -idirafter ${COMPILER_RESOURCE_DIR}/include
+.endif
+.endif
+
 CLANG_OPT_SMALL= -mstack-alignment=8 -mllvm -inline-threshold=3\
 		 -mllvm -simplifycfg-dup-ret
 .if ${COMPILER_VERSION} >= 30500 && ${COMPILER_VERSION} < 30700