Pass the --enable-libcpp configure option for cross builds, too.

<rdar://problem/15831288>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199413 91177308-0d34-0410-b5e6-96231b3b80d8

Makefile

@@ -114,9 +114,13 @@ cross-compile-build-tools:
 	  unset CXXFLAGS ; \
 	  unset SDKROOT ; \
 	  unset UNIVERSAL_SDK_PATH ; \
+	  configure_opts= ; \
+	  if test "$ENABLE_LIBCPP" -ne 0 ; then \
+	    configure_opts="$$configure_opts --enable-libcpp"; \
+	  fi; \
 	  $(PROJ_SRC_DIR)/configure --build=$(BUILD_TRIPLE) \
 		--host=$(BUILD_TRIPLE) --target=$(BUILD_TRIPLE) \
-	        --disable-polly ; \
+	        --disable-polly $$configure_opts; \
 	  cd .. ; \
 	fi; \
 	($(MAKE) -C BuildTools \