Explicitly set CXXSTD to c++11 for old C++ code using std::auto_ptr<>.

GCC 12 defaults to C++17 which removes (not just deprecates)
std::auto_ptr<>. Trying to use CXXSTD of c++03 doesn't work with
libc++ headers, but c++11 does.

Reviewed by: brooks, imp, emaste
Differential Revision: https://reviews.freebsd.org/D37531