Use the right macro name when checking whether libc++ is being used.

This fixes a copy-paste error introduced in r397043 ("Update the Qt4 ports to
4.8.7"): libc++ defines _LIBCPP_VERSION, not _LIBCPP_VER (which is defined by a
Dinkumware-based standard library used by QNX). In practice, this meant
Q_COMPILER_INITIALIZER_LISTS was never being enabled with clang.

Submitted by: Tobias Berner <tcberner@gmail.com>