innochecksum not built for --with-plugin-innodb_plugin --without-plugin-innobase

In 5.1, we can have the traditional "innobase" code
(built-in) or the new version "innodb" (plugin).
The help tool "innochecksum" is useful for both,
but its generation was coupled to "innobase" only.

Fix this by treating both "innobase" and "innodb"
equivalent in the configure phase,
this affects both "innochecksum" and the InnoDB documentation.

This patch was proposed by Mark Callaghan.