[ptx] enblend -z parameter fails ungracefully

[Bruno Postle]

Hugin calls enblend with this command-line:

  enblend -v -z

'-z' sets LZW compression on the output file.

..but I get a series of error messages:

  LZW compression is not available to due to Unisys patent enforcement.
  LZW compression is not available to due to Unisys patent enforcement.
  LZW compression is not available to due to Unisys patent enforcement.
  LZW compression is not available to due to Unisys patent enforcement.
  ...

The resulting tif file is empty and tiny.

I have libtiff-3.5.7.  Maybe -z shouldn't be the hugin default or
enblend should ignore -z when LZW is disabled in the library.

-- 
Bruno