Replace deprecated g_basename with g_path_get_basename.
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).
Add some missing casts to silence compiler warnings
Remove unused variables to silence compiler warnings
Add missing include

Silences:
  listen.c:320:3: warning: implicit declaration of function ‘ascii_strup’ [-Wimplicit-function-declaration]
Merge pull request #63 from dgl/cleanups

Clean-up some compiler warnings
Merge pull request #60 from dgl/perl-no-get-context

Define PERL_NO_GET_CONTEXT in all perl source files
Merge pull request #59 from ailin-nemui/perl-pl-na

get rid of PL_na
Merge pull request #44 from dajohi/g_basename

Replace deprecated g_basename with g_path_get_basename.