Allow blead to compile under some g++

Various platforms are refusing to compile blead with g++ [perl #121151]
This patch, suggested by Tony Cook seems to work.  There may be a better
way to do it, so I'm not closing the ticket, but this gets things
working again.

(cherry picked from commit e9d5f7d5c47a96d94ce10d3b5677a9625fe6c413)
patchlevel.h: we are now perl-5.18.3-RC2