regex-opt: use libstdc++

Fixes #26104.
Added new line.
InspIRCd 2.0.15

Closes #26108.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  InspIRCd 2.0.15
  regex-opt: use libstdc++