Add an option '--enable-libcpp' that will have the compiler pass on
options to use libc++ as the default c++ library.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144413 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/Makefile b/Makefile
index 016768e..cc47f0d 100644
--- a/Makefile
+++ b/Makefile
@@ -131,6 +131,7 @@
 	  ENABLE_COVERAGE=$(ENABLE_COVERAGE) \
 	  DISABLE_ASSERTIONS=$(DISABLE_ASSERTIONS) \
 	  ENABLE_EXPENSIVE_CHECKS=$(ENABLE_EXPENSIVE_CHECKS) \
+	  ENABLE_LIBCPP=$(ENABLE_LIBCPP) \
 	  CFLAGS= \
 	  CXXFLAGS= \
 	) || exit 1;