[krbdev.mit.edu #1588] We need --enable-maintainer-mode

[Sam Hartman via RT]

Many autoconf packages that support running autoconf automatically
default to not doing so and provide a --enable-maintainer-mode switch
to configure to enable this feature.

Krb5 should gain this functionality.  Without this functionality,it is
difficult to correctly store krb5 in a CVS repository or to apply
patches during an automated build process without breaking timestamps.