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/autoconf/configure.ac b/autoconf/configure.ac
index 6e9d848..090f5dc 100644
--- a/autoconf/configure.ac
+++ b/autoconf/configure.ac
@@ -415,6 +415,18 @@
 dnl===
 dnl===-----------------------------------------------------------------------===
 
+dnl --enable-libcpp : check whether or not to use libc++ on the command line
+AC_ARG_ENABLE(libcpp,
+              AS_HELP_STRING([--enable-libcpp],
+                             [Use libc++ if available (default is NO)]),,
+                             enableval=default)
+case "$enableval" in
+  yes) AC_SUBST(ENABLE_LIBCPP,[1]) ;;
+  no)  AC_SUBST(ENABLE_LIBCPP,[0]) ;;
+  default) AC_SUBST(ENABLE_LIBCPP,[0]);;
+  *) AC_MSG_ERROR([Invalid setting for --enable-libcpp. Use "yes" or "no"]) ;;
+esac
+
 dnl --enable-optimized : check whether they want to do an optimized build:
 AC_ARG_ENABLE(optimized, AS_HELP_STRING(
  --enable-optimized,[Compile with optimizations enabled (default is NO)]),,enableval=$optimize)