Correct casing of MySQL
MySQL 4.0 is not supported on RT 4.0; MySQL 4.1 is the minimum required

Combines two conditionals into one since they can both assume the
MySQL version is >= 4.1.
Check InnoDB support via SHOW ENGINES for MySQL 5.6 compatibility

MySQL 5.6 (still in release candidate stage) drops the 'have_innodb'
variable in favor of SHOW ENGINES.  The latter is supported since 4.1,
so we can safely use it across the board.

In my testing on 5.1, I never saw "disabled" when disabling InnoDB via
'skip-innodb'.  SHOW ENGINES always reported "no" despite the engine
being disabled at runtime not compile time.  Hint at this possibility in
the error message for "no".
Merge branch '4.0/mysql-innodb-check' into 4.0-trunk