Better handling of symlink creation

OpenBSD heavily uses the mksymlinks define to build outside of the src
tree.  This combines two commits from OpenBSD that improve the
robustness and speed of creating the symlinks.

From r1.24 by deraadt@:
> After the MANIFEST-based symbolic link creation loop, there is a check
> for a specific link to see if things worked out.  Add a check for the very
> last file in the MANIFEST, as well, since we are trying to spot a very
> odd bug where symbolic links are not being created.  Hopefully this will
> help us diagnose it.

http://www.openbsd.org/cgi-bin/cvsweb/src/gnu/usr.bin/perl/Configure#rev1.24

And from r1.25 by millert@:
> More efficient method of building the symlink tree that makes better
> use of awk.  Slightly faster and works around an apparent namei or
> buffer cache related bug on arm.

http://www.openbsd.org/cgi-bin/cvsweb/src/gnu/usr.bin/perl/Configure#rev1.25

Re-worked by Andy Armstrong to support the old awk used by systems
such as Solaris, see [perl #122002].