The assumption in the original commit for --builddir (648d3ced),
was to assume that without a --builddir, and when --srcdir
is specified, that the builddir is the same as the srcdir.

The problem is that this assumption does not hold for out-of-source
builds and we can figure out the builddir by looking for where
mysql_install_db script is.

As mysql_install_db is in the builddir, we use dirname0 as the builddir after
checking that my_print_defaults is also located from dirname0, otherwise
default to old behavior.