- fix warnings detected by -Werror=undef which appear when
   merging into 5.8
 - default to changing #if -> #ifdef
 - remove #ifdef'ed code which is obviously outdated, for example
   workaround for compiler versions more than 10 years old.
 - remove code for checking ZPAGE_STATE_POS whihćh is never
   defined or used in the code
 - remove code within ERROR_INSERT_CODE, that define is never defined