Mac: Automate build with libc++ when not using OPENSCAD_LIBRARIES

On MacOS 10.9 and later, Homebrew defaults to building all libraries linked against libc++. This detects a libc++ linked boost library in /usr/local and adds that flag as needed. This might make OpenSCAD build work out of the box on a mac using system-wide Homebrew installations of boost, CGAL, qt, etc. It should not affect builds using OPENSCAD_LIBRARIES.