Include path syntax on Mac

[Urs Liska]

Hi all,

it seems LilyPond on Windows requires paths passed to the -I command 
line option to have a trailing slash while LilyPond on Linux doesn't 
seem to care whether there is or not (see 
https://github.com/wbsoft/frescobaldi/issues/1094). It seems a fix to 
the issue in Frescobaldi is to ensure there's a trailing slash to the 
paths, which makes it work on Windows and Linux.

Before merging this code I'd need to know whether Mac also doesn't care 
about the trailing slash. Obviously it accepts paths without slashes, 
but if it should not accept them *with* slashes I'd need to make the 
changed code aware of the OS.

Could someone please test (or tell me directly) whether it makes a 
difference if a path passed to -I has a trailing slash or not on Mac?

Thanks
Urs