The Configure script apparently detects memrchr now, but uses -std=c99 by
default, so it isn't actually available (it is guarded with _DEFAULT_SOURCE).
Use -std=gnu99 explicitly instead.
Also, set the site and vendor man directories.
Otherwise, packages which install man pages will install them into $prefix/man.
(Although perl packages using ExtUtils::MakeMaker still don't install man pages, it seems.)
Put the man pages of perl itself into a doc subpackage.
753423afc5