perldelta - Remove spurious New Errors entries

Spotted by Father C - These are not new errors.  They are just listed with
explicit ‘exists’ and ‘delete’ now (and alphabetised under the same) rather
than %s.
Note that the USE_64_BIT_INT build option is not supported with MSVC++ 6.0

Also change the Windows makefiles to disable this option if it is set with
this compiler selected.  (Use !UNDEF (for nmake) and != (for dmake) to be
sure of disabling the option if it is defined on the command-line rather
than by editing the makefile.)