Fix build on MinGW with dmake

Add gmodule-2.0 to required libraries, as its used by GObjectIntrospection.xs.

Add a workaround for DMAKE to postamble().
Avoid misusing PL_na

We used to do "newSVpv (string, PL_na)" when we didn't know the string's
length.  That's not correct, and breaks if other XS modules write to PL_na,
which is what PL_na is intended for.  Simply use "newSVpv (string, 0)" instead.
Support unichar ↔ SV when invoking Perl code