Move the definition of SVt_LAST before its first use
Disable OURSTASH retrieval if SvOURSTASH is not defined

This macro is not available on 5.8, where the stash for our variable can
be retrieved only if you have the relevant lexical pad pointer and offset.
GvNAME_get is not defined on 5.8

On more recent perls it's #defined as GvNAME, so use that instead.
On 5.8, PERL_UNUSED_VAR cannot take a const variable

because it's actually defined as 'if (0) var = var'. So
drop the const to make it compile.
Merge pull request #5 from rgs/five-eight

Five eight