Define PERL_NO_GET_CONTEXT in all perl source files

This removes the calls to Perl_get_context() that get automatically
added to XS code for ancient source code compatibility reasons.

The result is about a ~60K size reduction in the binary (based on
comparing two 64-bit stripped irssi binaries compiled
--with-perl-staticlib).
Merge pull request #60 from dgl/perl-no-get-context

Define PERL_NO_GET_CONTEXT in all perl source files