Thank you for your post, It helped me compiling the DBD::Sybase (v.1.15) with FreeTds. However, the next error I've encountered was:

Can't find any Sybase libraries in /usr/local/lib or /usr/local/lib64 
+at Makefile.PL line 155, <IN> line 44.
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I had to change

  my @libdir = ( 'lib', 'lib64' );
  if ($^O =~ /win/i) {
    @libdir = ( 'dll' );
  }
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to

  my @libdir = ( 'lib', 'lib64' );
  if ($^O =~ /^win/i) {
    @libdir = ( 'dll' );
  }
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(cygwin contains "win" , then the Makefile is looking for "dll" library subfolder, instead of "lib"/"lib64")