bug#62657: PCRE2-related workarounds that GNU grep might need

[Paul Eggert]

Recent commits in Git do the following to work around bugs in PCRE2. 
Quite possibly GNU grep -P should do the same, when in a UTF-8 locale.

  * Disable PCRE2_UCP unless PCRE2 10.35 or higher.
  * If ignoring case and PCRE2_MATCH_INVALID_UTF is defined, then 
enable PCRE2_NO_START_OPTIMIZE unless PCRE2 10.36 or higher.