darwin/cross-build: Unset SDKROOT when building BuildTools.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162402 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/Makefile b/Makefile
index 604696a..a935df4 100644
--- a/Makefile
+++ b/Makefile
@@ -111,6 +111,7 @@
 	  cd BuildTools ; \
 	  unset CFLAGS ; \
 	  unset CXXFLAGS ; \
+	  unset SDKROOT ; \
 	  $(PROJ_SRC_DIR)/configure --build=$(BUILD_TRIPLE) \
 		--host=$(BUILD_TRIPLE) --target=$(BUILD_TRIPLE) \
 	        --disable-polly ; \