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.
Change-Id: I5ef33b34b6a4e6111983a63a2d34326ca4af9d30
Reviewed-on: https://gerrit.openafs.org/13936
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>