Centralise the minimum-required-php-version in a MW_MIN_PHP_VERSION constant in Defin...

Centralise the minimum-required-php-version in a MW_MIN_PHP_VERSION constant in Defines.php.  This requires loading Defines.php before the PHP4 version checks, so include a big note reminding people not to include anything other than vanilla define() statements in there.

I have a sneaking suspicion that including Defines.php in the entry points might not play nicely with HipHop, but I can't test it (wrong OS).