FreeBSD support for Python bindings

In building the Ice bindings on FreeBSD, I ran into a couple of issues:

1) Use of const iconv in IconvStringConverter.h. This appears fixed on the git master branch, but still affects 3.6.2.

2) Use of incorrect logic in setup.py. The conditionals are effectively:

if platform is 'darwin':
  ...
elif platform is 'linux':
  ...
elif platform is 'windows'
  ...

However, if you're not one of these three platforms, the build breaks. The patch adjusts this to be:

if platform is 'darwin':
  ...
elif platform is 'windows'
  ...
else
  (generic unix/linux, with freebsd special case for libdl)

This will make the build work on a number of additional platforms by using the linux case as the general unix case. I couldn't see setup.py in ice git, so I'm not sure where this would need fixing. A proper feature test for libdl would make this truly generic.

See also: https://www.openmicroscopy.org/community/viewtopic.php?f=5&t=8100&p=17392#p17385

Kind regards,
Roger