Whitespace cleanup.
Fix 'make install' and 'make uninstall'.

Install spl.kext to /System/Library/Extensions.  This should be made
configurable later, but probably isn't likely to change.
Target SPL_AC_CONFIG autoconf glue for xnu.
Include SPL_AC_CONFIG and fix _puts symbol issues.

It looks like _puts appears as an undefined symbol for files that #include
OS X specific kernel headers, if they also use printf().  Fix those specific
places (spl-osx.c and spl-vnode.c) to use IOKit's IOLog() instead.  All uses
of printk and printf probably should be replaced with IOLog().

Including SPL_AC_CONFIG gives us configure-level access to being able to run
debug builds, which make use of ZFS's extensive assertion-based debugging
and verification system.