macos: tell the compiler the system include path

macos: tell the compiler the system include path

In order to support multiple SDKs, macOS Catalina no longer has the
/usr/include directory. As a result, the compiler needs to know where
these headers can be found. To successfully build OpenAFS on OSX 10.15,
set KROOT so the compiler knows the correct location of these headers.

Reviewed-on: https://gerrit.openafs.org/13936
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 677b038814817defec9421e698ce67b44a7fd7d1)

Change-Id: I2043c2bc6e745ca55faf68b77d791168bc57bb1d
Reviewed-on: https://gerrit.openafs.org/14031
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>