povray 3.7.0.RC6 squashed

Note: this pull request is a squashed version of the commits in issue #15093.

The previous version of povray (3.6.1) would not compile with recent versions of libpng. This beta version does compile with recent versions of libpng.

Almost all changes to this formula come from studying the [povray portfile][1] from MacPorts and reading [a bug report][2] about how they solved the same problem.

Note: povray now requires at least Lion to compile.

[1]: https://svn.macports.org/repository/macports/trunk/dports/graphics/povray/Portfile "MacPorts POV-Ray portfile"
[2]: https://trac.macports.org/ticket/35949 "MacPorts Ticket 35949"