Reported by the AdoptOpenJDK folks.

(https://github.com/AdoptOpenJDK/openjdk-build/issues/181)

"WARNING: Your build output directory is not on a local disk.
This will severely degrade build performance!
It is recommended that you create an output directory on a local disk,
and run the configure script again from that directory."

appears even though we build locally on AIX.

Check uses "df -l", AIX df does not know -l.