--stdlib is not respected

[dimpase]

this makes building with non-default C++ lib (e.g. clang's libc++) impossible, see
https://trac.sagemath.org/ticket/24707

[rwst]

In the course of that ticket it turned out that a a simple solution is to add --stdib=libc++ to LDFLAGS (and not as part of CXX). So I think this can be closed.

[malb]

Thanks for the update, closing :)