Make sure we aren't in generated Makefile.PL

The PERL5OPT=-MInline::Module=postamble... adds a postamble handler if called
from within a Makefile.PL. This might be the Makefile.PL that Inline
generates.

In general, we need a way to make the PERL5OPT thing only react to very
specific perl invocations. Perl gets called even from `git` sometimes and can
cause issues.
The makestub dance `use`s the stub module

The import should be a no-op in that case
Add CPP specific parser to `using`