par_streamlines does not compile using cl.exe

Hey, I'm using par_streamlines in my creative coding sketches and I noticed par_streamlines throws errors when compiling with cl.exe. It appears the errors are from the use of `_attribute__((unused))` in 4 spots, lines: 837, 867, 983, and 1009. Commenting these out seems to allow me to compile the library with no issues.

I can submit a merge request if that will help!