Build failure when scons uses python 3

As [reported to MacPorts](https://trac.macports.org/ticket/66973), field3d does not build when scons uses python 3:

```
scons: Reading SConscript files ...
  File "BuildSupport.py", line 276

    print cmd

    ^^^^^^^^^

SyntaxError: Missing parentheses in call to 'print'. Did you mean print(...)?
```
