CMakeLists.txt: don't build FUSE unless on Linux
CMakeLists.txt: don't build FUSE unless on Linux

FUSE is a Linux thing!  Although MacOS has a different project with the
same name.

Signed-off-by: Colin McCabe <3142ce8a0bdba4a38073365893e6b74f72d9c512@alumni.cmu.edu>

cmccabe/redfish,cmccabe/redfish,cmccabe/redfish,cmccabe/redfish,cmccabe/redfish