Check out illumos-gate on OS X

The illumos-gate repository (and presumably other UNIX projects) has some files that differ only in case (e.g. Hb and HB in the same directory -- see this issue). You can create a sparse (i.e. thinly provisioned) read-write disk image with case sensitivity turned on with these options to Disk Utility:

Mount this and clone repositories into it (it will be under /Volumes by default).