Fix for Perl 5.14

Perl 5.14.0-RC1 came out a few days ago...

There is a minor compile time error due to the API changing a bit:
plperl.c:929:3: error: lvalue required as left operand of assignment

This is due to GvCV() no longer returning an lvalue, instead they want
us to use the new GvCV_set macro. (see
http://search.cpan.org/~jesse/perl-5.14.0-RC1/pod/perldelta.pod#GvCV()_and_GvGP()_are_no_longer_lvalues)

Unfortunately  that macro is not available on older perls so the
attached provides our own macro when GvCV_set is not defined.

Tested with 5.14.0-rc1 and 5.12.3.

The -head patch applies with fuzz to 9.0. The 8.4 patch applies clean
to 8.4 and with fuzz to 8.3 and 8.2.