#54 src/linux/hip_linux_umh.c built on Mac OS X

[Jeff Ahrenholz]

low priority - the Mac OS X build of HIP requires src/linux/hip_linux_umh.c
both the filename and directory name are misleading; either split out separate functionality to src/mac/hip_mac_umh.c (there are many #ifdef __MACOSX__ in init_hip() for example) or rename and move the file to src/usermode/