Log Message:
pcre2: update to 10.31.

Version 10.31 12-February-2018
------------------------------

This is mainly a bugfix and tidying release (see ChangeLog for full details).
However, there are some minor enhancements.

1. New pcre2_config() options: PCRE2_CONFIG_NEVER_BACKSLASH_C and
PCRE2_CONFIG_COMPILED_WIDTHS.

2. New pcre2_pattern_info() option PCRE2_INFO_EXTRAOPTIONS to retrieve the
extra compile time options.

3. There are now public names for all the pcre2_compile() error numbers.

4. Added PCRE2_CALLOUT_STARTMATCH and PCRE2_CALLOUT_BACKTRACK bits to a new
field callout_flags in callout blocks.