Auto installdirs

Me again.

One problem with this solution to setting installdirs:

  installdirs   => ($] >= 5.009004 ? 'core' : 'site'),   #A

is that an auto-generated Makefile.PL will get a static setting for
INSTALLDIRS.  This is of course the problem with any dynamic settings
in Build.PL that are expected to propagate to Makefile.PL (or META.yml
for that matter) - they get frozen at `Build dist` time.

One way to solve this particular item would be like so:

  installdirs_auto   => 5.009004,  #B

which would effectively function like #A, and generate in the Makefile.PL:

  INSTALLDIRS   => ($] >= 5.009004 ? 'perl' : 'site'),  #C

Now, does that do everything we'd want?  What about the 'vendor' dirs?
 Are those ever even fricking used anyway?

 -Ken