Make the library play nicer with includes when using the Slash library as a peer in a workspace.

Public Headers Path needs to be "include/Slash". As well, you need to have all headers under "Public" in "Copy Headers" also in a separate "Copy Files" build phase, with "Absolute Path" set to "../BuildProductsPath/$(CONFIGURATION)-$(PLATFORM_NAME)/include/Slash"

This seems to be the only bulletproof way to make sure it works in all configurations, no matter whether it's a subproject or a workspace.