Rewrite FUSE macro to skip FUSE if not found unless --enabled

Rewrite FUSE macro to skip FUSE if not found unless --enabled

If no configure flags are given, build the FUSE afsd iff FUSE is
found.  If --enable-fuse-client is explicitly given, always try to
build the FUSE client and abort if FUSE libraries could not be found.

Change-Id: Icc0d5bbb03239470ad856d87bca600304f030591
Reviewed-on: http://gerrit.openafs.org/2419
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>