Set EXAMPLE_CONF_DIR to DOCDIR if DOCDIR gets customized

Set EXAMPLE_CONF_DIR to DOCDIR if DOCDIR gets customized

Before, customisation of DOCDIR did not lead to
changes of the EXAMPLE_CONF_DIR default value.
This was inconsistent with expected behaviour
if you didn't know of the existence of the
EXAMPLE_CONF_DIR variable. The confusion is
very strong, as (for not in place builds) both
DOCDIR and EXAMPLE_CONF_DIR share the same
default value, it is unexpected that setting
CUSTOM_DOCDIR changes the location of all
documentation files but one.

The best meaning of EXAMPLE_CONF_DIR is to
further allow customization of the mt.conf.example
storage place, but otherwise keep defaulting to
whatever DOCDIR is set to, at least for not in
place builds. This patch implements that
behaviour.

Fixes #3863.