Noticed missing target directory dependency as a build failure in
`make --shuffle` mode (added in https://savannah.gnu.org/bugs/index.php?62100):
CC test_common.o
In file included from test_common.c:34:
krb5/gsskrb5_locl.h:42:10: fatal error: gkrb5_err.h: No such file or directory
42 | #include <gkrb5_err.h>
| ^~~~~~~~~~~~~
compilation terminated.
make[3]: *** [Makefile:2347: test_common.o] Error 1 shuffle=1656680590
The change moves gkrb5_err.h and friends to BUILT_SOURCES
to guarantee their presence when main build starts.
e7efa7783a