Revert to a somewhat safer krb5 test (hopefully working for MIT
as well as Heimdal now). If we do not even have the krb5_is_thread_safe()
function, disable OpenMP for krb5-18/23 formats to be safe.
Don't try to use HAVE_LIBKRB5 as an aggregate - create a new one.
Merge pull request #625 from frank-dittrich/bleeding-jumbo

Use "$(CC)" instead of "gcc" in Makefiles
some more Makefile changes

for make clean, use the parent's jobserver for subdirectories
redirect some unwanted possible stderr output to /dev/null