Inline the Makefile rule for extra.install into all 6 targets which use it.

This eliminates 6 recursive calls from Makefile to itself. As five of these
are generated from a loop in Makefile.SH, this isn't massive code
duplication. It's not obvious how to merge the sixth without increasing the
code complexity.

Additionally, if the user has specified extra modules to download and build
from CPAN using Configure's -Dextra, this avoids building perl, the non-XS
modules and the CPAN modules twice on make install.