Bug: please improve selection of C++ standard library on Mac OS

[William Gallafent]

Hello there!

As per https://trac.macports.org/ticket/48627 there's a wrinkle when
building ncurses 6.0 on Mac OS when one wishes to link libc++ as the
C++ runtime, instead of libstdc++.

I hope there's a simple way to improve the selection of this runtime
in the configure script: clearly the hack I suggest there makes things
work for me when linking libc++ but would break things for anybody
wishing to link libstdc++, so it's not a solution, just a
quick-and-dirty workaround!

-- 
Bill Gallafent.