Add libc++ version detection.

[SVN r83925]

test/config_info.cpp

@@ -563,6 +563,8 @@ void print_stdlib_macros()
    PRINT_MACRO(_HAS_EXCEPTIONS);
    PRINT_MACRO(_HAS_MEMBER_TEMPLATES_REBIND);
    PRINT_MACRO(_HAS_TEMPLATE_PARTIAL_ORDERING);
+   // Libc++:
+   PRINT_MACRO(_LIBCPP_VERSION);
    // STLPort and generic SGI STL options:
    PRINT_MACRO(__SGI_STL_NO_ARROW_OPERATOR);
    PRINT_MACRO(__SGI_STL_OWN_IOSTREAMS);