oniguruma: bump to 6.9.5_rev1, switch to cmake

oniguruma: bump to 6.9.5_rev1, switch to cmake

Besides the version bump, this backports a patch so that cmake uses the
same so version information as the current autotools, allowing the
switch over to cmake.

6.9.5_rev1 Changelog:

- Look-behind bug: if the look-behind contains a branch with a character
  length of 0 and an anchor is included in the branch, the whole
  look-behind is ignored
- POSIX API disabled by default -- enabled in openwrt, as at least
  libevhtp needs it
- Update Unicode version 13.0.0
- NEW: Code point sequence notation \x{HHHH HHHH ...}, \o{OOOO OOOO ...}
- NEW API: retry limit in search functions
- NEW API: maximum nesting level of subexp call
- Fixed behavior of isolated options in Perl and Java syntaxes.
  /...(?i).../

Signed-off-by: Eneas U de Queiroz <redacted>